Late Ottawan Orogenic Collapse of the Adirondacks in the Grenville Province of New York State (USA): Integrated Petrologic

Research Paper

GEOSPHERE Late Ottawan orogenic collapse of the Adirondacks in the Grenville province of New York State (USA): Integrated petrologic,

GEOSPHERE, v. 16, no. 3 geochronologic, and structural analysis of the Diana Complex in https://doi.org/10.1130/GES02155.1 the southern Carthage-Colton mylonite zone 23 figures; 5 tables Graham B. Baird CORRESPONDENCE: [email protected] Department of Earth and Atmospheric Sciences, University of Northern Colorado, Campus Box 100, Greeley, Colorado 80639, USA

CITATION: Baird, G.B., 2020, Late Ottawan orogenic collapse of the Adirondacks in the Grenville prov- ABSTRACT formed and the CCMZ was rotated into is current kinematic indicators associated with mylonitic ince of New York State (USA): Integrated petrologic, orientation and overprinted with other structures. fabrics in the south-central part of the zone (Baird, geochronologic, and structural analysis of the Diana Crustal-scale shear zones can be highly import- 2006); (3) transcurrent Ottawan-aged ductile fab- Complex in the southern Carthage-Colton mylonite zone: Geosphere, v. 16, no. 3, p. 844–874, https://doi ant but complicated orogenic structures, therefore rics in the central part of the zone (Johnson et al., .org/10.1130/GES02155.1. they must be studied in detail along their entire ■■ INTRODUCTION 2004); (4) the apparently folded and steep character length. The Carthage-Colton mylonite zone (CCMZ) of the zone in its northern half (Fig. 1B; Johnson Science Editor: Andrea Hampel is one such shear zone in the northwestern Adiron- Crustal-scale shear zones are fundamental et al., 2004); and (5) the zone’s relationship to the Associate Editor: Francesco Mazzarini dacks of northern New York State (USA), part of features of orogens. They allow juxtaposition of dif- region’s cooling pattern and other structures inside the Mesoproterozoic Grenville province. The south- ferent terranes or crustal blocks, can be important and outside of the zone (Fig. 1C). Received 17 May 2019 Revision received 5 December 2019 ern CCMZ is contained within the Diana Complex, fluid conduits, and accommodate minor to signif- This contribution extends the work presented Accepted 10 February 2020 and geochemistry and U-Pb zircon geochronology icant crustal deformation of any and/or multiple in Baird (2006) and tests whether a lithological demonstrate that the Diana Complex is expansive kinematics (e.g., Hanmer and McEachern, 1992; van discontinuity exists across a portion of the zone. Published online 19 March 2020 and collectively crystallized at 1164.3 ± 6.2 Ma. der Pluijm et al., 1994; Mahan et al., 2006). Decipher- Further, this work fleshes out aspects of a rotated Major ductile structures within the CCMZ and ing their tectonic history can be exceedingly difficult detachment model for the CCMZ presented in Baird Diana Complex include a northwest-dipping penetra- because varying and commonly repeating tectonic (2008) and incorporates the recent high-quality tive regional mylonitic foliation with north-trending processes localized along shear zones can eradicate microstructural and isotopic results of Bonamici lineation that bisects a conjugate set of mesoscale the evidence needed to interpret their history. The et al. (2011, 2014, 2015). The tectonic history of the ductile shear zones. These ductile structures formed Carthage-Colton mylonite zone (CCMZ; Geraghty adjacent terranes is also considered, resulting in from the same 1060–1050 Ma pure shear transition- et al., 1981; Fig. 1) is one such fundamental crust- the construction of tectonic models that propose ing to a top-to-the-SSE shearing event at ~700 °C. al-scale shear zone. Located in the northwestern solutions to any apparent discrepancies with vari- Other important structures include a ductile fault Adirondacks (northern New York State, USA), a able Ottawan-aged kinematic indicators identified and breccia zones. The ductile fault formed imme- southern exposure of the Mesoproterozoic Gren- along the structure. This work further emphasizes diately following the major ductile structures, while ville province (e.g., Rivers, 2008; McLelland et al., the complexity of crustal-scale shear zones and the the breccia zones may have formed at ca. 945 Ma 2010), the complete history and significance of the need to study such zones by integration of struc- in greenschist facies conditions. CCMZ remains unclear. The structure’s later history tural, petrologic, and geochronologic data sets Two models can explain the studied struc- does include tectonic collapse at the end of the along their entire length. tures and other regional observations. Model 1 Ottawan orogeny (ca. 1060–1030 Ma; Selleck et al., postulates that the CCMZ is an Ottawan orogeny 2005) and exhumation of the Adirondack Highlands, (1090–1035 Ma) thrust, which was later reactivated the terrane to the southeast of the zone (Fig. 1). ■■ GEOLOGIC BACKGROUND locally as a tectonic collapse structure. Model 2, the However, a number of key zone characteristics are preferred model, postulates that the CCMZ initially not yet understood in this extensional model for the The Grenville province, broadly described as formed as a subhorizontal mid-crustal mylonite CCMZ. These include: (1) the lack of an obvious lith- a series of terranes separated by complex crust- This paper is published under the terms of the zone during collapse of the Ottawan orogen. With ological discontinuity in sections of the zone; (2) the al-scale shear zones (e.g., Rivers et al., 1989), occurs CC‑BY-NC license. continued collapse, a metamorphic core complex identification of sinistral oblique reverse–sensed in northern New York State as the Adirondacks

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(Fig. 1). In general, the Adirondack segment of the Accreted Terranes <1.3 Ga 45o o Grenville province formed by protracted accretion- Ottawan and older o Carthage-Colton 74 75 ary, collisional, and magmatic events ranging in age orogenic signatures Mylonite Zone from ca. 1.35 Ga to ca. 0.98 Ga (Rivers, 2008; McLel- Lacks Ottawan land et al., 2010). However, recent work within the orogenic Grenville province also emphasizes the importance signature N of tectonic collapse late in its history, as exemplified Grenville Province by work in the Adirondacks, the Morin terrane, the MTD east side of the Mékinac-Taureau domain, and the DH ORGC MT Ottawa River Gneiss Complex (Selleck et al., 2005; B Wong et al., 2011; Rivers and Schwerdtner, 2015; Soucy La Roche et al., 2015; Schwerdtner et al., MM New York DC 2016; Dufréchou, 2017; Regan et al., 2019). Adirondack Fig.2 44o Two terranes are exposed in the Adirondacks, Lowlands o Adirondack

the Adirondack Lowlands (Lowlands) and the 76 IRR Adirondack Highlands (Highlands), which are sep- Highlands arated by the CCMZ (Fig. 1B). The geology of the Lowlands includes upper amphibolite grade (~640– CCMZ 1050 680 °C, ~6.0–7.8 kbar; Bohlen et al., 1985; Kitchen and Valley, 1995), complexly folded metasedimen- 1000 G&P tary rocks and some metaigneous and igneous Piseco Lake Shear Zone rocks (e.g., McLelland and Isachsen, 1986). Many Gb & Am EASZ 950 ductile shear zones exist throughout the Lowlands, Cooling Age (Ma) A S. Adks km but little detailed work has focused on these struc- NW SE Gr 0 10 20 30 40 tures, so nearly all have unknown deformation -50 0 125 DC & S LSZ 43o histories (Carl and deLorraine, 1997; Baird and Dist. Perpendicular to CCMZ (km) P HAT Hbl Ar/Ar Shrady, 2011). Highlands geology is generally sim- Figure 1. (A) Overview of the Grenville province (southeastern Canada and northeastern United States) with the Adirondacks ilar to that of the Lowlands but includes a higher, highlighted within the green polygon (after Rivers, 2008; Baird and Shrady, 2011). ORGC—Ottawa River Gneiss Complex granulite facies metamorphic grade (~675–850 °C, (Schwerdtner et al., 2016); MT—Morin terrane; MTD—Mékinac-Taureau domain. (B) General lithology and features of the Adirondacks (after Isachsen and Fisher, 1970; McLelland and Isachsen, 1980; Carl and deLorraine, 1997; Geraghty et al., 1981; ~6.5–9.0 kbar; Bohlen et al., 1985; Kitchen and Valley, Wong et al., 2011; Valentino et al., 2018). IRR—Indian River Road; DC—Diana Complex, study area outlined (Fig. 2); DH—Dana 1995; Spear and Markussen, 1997; Storm and Spear, Hill metagabbro (circled); MM—Marcy massif anorthosite complex; EASZ—East Adirondack shear zone; S. Adks—southern 2005) and a higher proportion of metaigneous and Adirondacks; LSZ—Adirondack Lowlands shear zones; HAT—axial traces of major Adirondack Highlands folds; G&P—glacial deposits and Paleozoic rocks; Gb & Am—gabbroic and amphibolitic rocks; A—anorthositic rocks; Gr—granitoids; DC & S— igneous rocks relative to metasedimentary rocks Diana complex and other syenitic rocks; P—paragneisses including marble, calc-silicates, quartzite, and biotite gneisses; Hbl (e.g., McLelland and Isachsen, 1986). Multiple gen- Ar/Ar—sites of hornblende 40Ar/39Ar geochronology (Onstott and Peacock, 1987; Heizler and Harrison, 1998; Streepey et al., erations of folding are also evident in the Highlands 2000, 2001, 2004; Dahl et al., 2004). (C) Approximately northwest-southeast transect showing reported hornblende 40Ar/39Ar ages in the Adirondack Lowlands (negative distance), the Carthage-Colton mylonite zone (CCMZ, red bar), and Adirondack and produce the terrane’s dominant structural grain Highlands (positive distance). Distances are measured by cross-strike distance to the center of the CCMZ. Gray band high- that follows major fold axial traces (McLelland and lights the general trend of most ages. Dist.—distance. Isachsen, 1980; Fig. 1B). A tectonic model outlining Adirondack geology is provided by McLelland et al. (2010; 2013, and following and partially overlapping the Shawinigan CCMZ and the Lowlands, some argue that Shawin- references therein). Major events include magma- events was widespread intrusion of the anortho- igan-aged thrusting may have occurred along the tism, metamorphism, and deformation associated site-mangerite-charnockite-granite magmas (AMCG CCMZ (Baird and Shrady, 2011; McLelland et al., with the Shawinigan orogeny, which was driven by suite) thought to have been created by mantle and 2013). Subsequently, the collision of Amazonia with the closure of a complex backarc basin now found crustal partial melting initiated by delamination or Laurentia produced the Ottawan orogeny (ca. 1090– as forming the Lowlands and perhaps portions slab breakoff (ca. 1160–1145 Ma; McLelland et al., 1035 Ma; McLelland et al., 2001). This event only of the Highlands (ca. 1200–1150 Ma; Chiarenzelli 1996, 2004; Regan et al., 2011). Based on similar- affected the Adirondack Highlands, because the et al., 2010; Baird and Shrady, 2011). Immediately ity of deformation style between portions of the Adirondack Lowlands were part of an orogenic

lid, or a region shallower in the crust that escapes melting (e.g., Vanderhaeghe and Teyssier, 1997). ductile lower crust. This would have led to greater later high-grade metamorphism, magmatism, and The gravitational instability is alleviated by brittle vertical thinning and extension and more compli- ductile deformation experienced by lower-crustal extension in the upper crust and exhumation of cated strain patterns upon collapse. rocks such as in the Highlands (Fig. 1; Rivers, 2008). ductile lower crust, which in turn may generate Recognition of the Lowlands as part of an orogenic (more) melt through decompression (Teyssier and lid emphasizes that despite the apparent similarity Whitney, 2002). The Carthage-Colton Mylonite Zone of geology found in the Lowlands and Highlands, The characteristic feature of metamorphic core the difference in tectonic history suggests that complexes is generally dome-shaped ductile low- Geraghty et al. (1981) defined the CCMZ as a appropriate correlation of pressure-temperature er-crustal rocks overlain by an extension-dominated zone of high strain and mylonitic textures in the conditions and deformation features between the brittle to ductile shear zone, commonly called the northwestern Adirondacks. Mezger et al. (1991, two terranes always requires temporal constraints. detachment. Above the detachment is normal-faulted 1992) first demonstrated the approximate 100 m.y. Orogenic collapse of the Ottawan orogen occurred upper crust (e.g., Whitney et al., 2013). Though met- difference in U-Pb sphene cooling ages on either synchronously with intrusion of the Lyon Mountain amorphic core complexes can be symmetric, as side of the CCMZ in the Highlands (ca. 1050– Granite Gneiss into the Adirondack Highlands and proposed for the Adirondacks (Isachsen and Geraghty, 990 Ma) and Lowlands (ca. 1150–1090 Ma). This, extension along both the CCMZ and East Adiron- 1986; Bickford et al., 2008; Wong et al., 2011), most are combined with the impossibly high thermal gradi- dack shear zone at ca. 1060–1030 Ma (Selleck et al., asymmetric with one dominant detachment. If the ents found across the zone, indicates that the CCMZ 2005; Bickford et al., 2008; Wong et al., 2011; Chi- lower crust was migmatitic at the time of exhuma- is a terrane boundary and experienced one or more arenzelli et al., 2017; Fig. 1B). tion, the lower crust would commonly have granitic kilometers to tens of kilometers of shearing events plutons and complicated foliation patterns defining across it (Mezger et al., 1992). Other workers, using multiple sub-domes (e.g., Whitney et al., 2004, 2013). different isotopic and minerals systems (Streepey Metamorphic Core Complexes: Geological The lower crust may also include a mid-crustal et al., 2000, 2004; Dahl et al., 2004), defined sepa- Overview mylonite zone that is not the detachment; however, rate cooling paths for the Highlands and Lowlands, this mylonite zone is warped over and through the which helped shape tectonic models explaining The Adirondacks may be part of a deeply eroded exhumed dome, merging with and overprinted by that through CCMZ shearing, both terranes expe- symmetric metamorphic core complex, as the High- the detachment on one side of the core complex rienced Shawinigan tectonism, while only the lands are bound by the generally northwest-dipping (e.g., Lister and Davis, 1989; Cooper et al., 2010; Highlands experienced Ottawan tectonism. normal-sensed CCMZ and the ~20° southeast–dip- Platt et al., 2014). This mid-crustal mylonite zone Figure 1C summarizes how the 40Ar/39Ar horn- ping normal-sensed East Adirondack shear zone is the ductile sole of upper-crustal normal faults blende age young to the southeast across the (Isachsen and Geraghty, 1986; Bickford et al., 2008; produced by extension preceding doming. Lister Lowlands from ca. 1100 Ma, to a range of ages (ca. Wong et al., 2011). In order to fully appreciate major and Davis (1989) termed this zone the “mylonite 1025–925 Ma) within the CCMZ, and reaching an features, characteristics, and processes associated front”, while Cooper et al. (2010) called this zone approximately consistent young age of ca. 950– with metamorphic core complexes and how they the “localized-distributed transition.” Depending on 900 Ma in the Highlands. These data suggest that might fit with the deformational and metamorphic the geometry and history of the normal faults that the Lowlands were either tilted to the northwest after history of the CCMZ presented here, it is worthwhile sole into this zone, both kinematics and total dis- cooling, or cooled toward the southeast via inclined to provide an outline of the relevant features associ- placement can be quite variable along the length of isotherms. Further, the Highlands were exhumed ated with crustal extension and metamorphic core the mid-crustal mylonite zone (Cooper et al., 2010). by one or more episodes of normal-sensed motion complex generation. Many sources provide excel- The Adirondack Highlands and other meta- along the CCMZ in the ca. 1100–940 Ma interval lent reviews and detailed studies of metamorphic morphic core complex examples proposed for (Dahl et al., 2004). core complexes (e.g., Lister and Davis, 1989; Whitney the Grenville province do differ from the classic Streepey et al. (2001) and Johnson et al. (2004, et al., 2013; Platt et al., 2014), which are a widespread Mesozoic examples of the western United States. 2005) were the first to provide complete struc- crustal structure and have formed throughout much Most obvious is the greater size of the Highlands tural, petrologic, and geochronologic analysis of of Earth history (e.g., Whitney et al., 2013). in comparison. This difference could be simply due the Dana Hill metagabbro segment of the CCMZ A general model for metamorphic core complex to deeper exposure of the Proterozoic complexes (Fig. 1B). Results indicate that early ductile struc- formation includes an initial period of crustal thick- producing a greater aerial size. Rivers (2011) also tures resulted from dextral strike-slip motion under ening. This thickened crust become gravitationally argues that Proterozoic orogens, the Grenville ~740 ± 30 °C conditions at ca. 1090–1050 Ma. This unstable as the lower crust becomes weak in a few province in particular, were much hotter compared was followed by normal-sensed dip-slip defor- tens of millions of years due to heating and partial to more recent examples and resulted in a more mation at 1050–1020 Ma during fluid infiltration

at ~700 °C. Deformation ended in the Dana Hill and typical marbles, quartzites, and related rocks of to offsets across the CCMZ after AMCG suite metagabbro with brittle deformation at greenschist the Lowlands is a multiply folded intrusive contact intrusion, bulk-rock geochemistry of one repre- facies conditions sometime during 1000–930 Ma. (Wiener, 1983; Fig. 2). Wiener (1983) suggested that sentative sample each of pyroxene-bearing and The Diana Complex occurs along the south- the Diana Complex intruded synchronously with the hornblende-bearing Diana Complex units and of ern CCMZ and, as mapped by Hargraves (1969), second folding event identified in the Lowlands adja- the aplitic granite gneiss was studied. Rock anal- extends across the zone and into both the Lowlands cent to the complex. Hargraves (1969), building on ysis was performed by the mineral laboratories and the Highlands. Therefore, it is unclear whether the work of Buddington (1939), identified two main division of Bureau Veritas Mineral Laboratories the complex is a Lowlands or a Highlands unit, and lithologies for the Diana Complex based on the dom- (Acme Labs; Vancouver, British Columbia, Canada) no recent work has investigated whether the cur- inant accessory mineralogy: either predominately (Table 1). Analysis procedure included pulverization rently identified Diana Complex is indeed a single pyroxene-bearing or predominately hornblende- of ~650 g of rock, followed by lithium borate fusion, magmatic complex. If large displacements occurred bearing. Pyroxene-bearing units tend to be green acid digestion, and analysis by inductively coupled across the CCMZ, a lithological discontinuity should in outcrop while hornblende-bearing rocks tend to plasma–mass spectrometry (trace elements) and exist along all portions of the CCMZ, but this has be pink. Cartwright et al. (1993) and Lamb (1993) col- inductively coupled plasma–optical emission spec- not been tested in the Diana Complex. lectively reported that clinopyroxene, orthopyroxene, trometry (major elements). The complexity of deformation and resulting and hornblende are in variable proportions at most Normative analysis of these analyses and of implications within part of the Diana Complex locations within the Diana Complex. additional Diana Complex bulk chemical analyses segment of the CCMZ were considered by Baird Smyth and Buddington (1926) described the presented by Smyth and Buddington (1926), Bud- and MacDonald (2004), while Bonamici et al. (2011, Diana Complex rocks as syenitic because other dington (1939), and Hamilton et al. (2004) were 2014, 2015) provided a robust microstructural and workers of the time used such terminology when identically calculated by a Microsoft Excel spread- isotopic analysis of sphene from two expansive describing similar rocks throughout the Adiron- sheet constructed by K. Hollacher (http://minerva. outcrops in this area (“east,” here called H1; and dacks. This classification is nearly universally used union.edu/hollochk/c_petrology/other_files/norm4. “west”; Fig. 2). Results included identification of today but is not without issues given the reinte- xlsx) following the routine of Johannsen (1931). two shear fabrics that crosscut a penetrative defor- grated ternary composition (Fuhrman et al., 1988; Results are presented in Figure 3A. For consistency, mation fabric. These shear fabrics formed under Lamb, 1993) of the mesoperthite that currently all analyses shown in Figure 3A assumed only Fe2+ rapidly cooling conditions from ~700 to 500 °C in dominates the rock. Other similar pyroxene-bearing was present. Some analyses reported Fe3+ content; a few million years at ca. 1050 Ma. All fabrics were rocks are classified, depending on technique, as inclusion of Fe3+ in the normative analysis produced presumed to have formed during peak Ottawan mangerite-charnockite, monzonite, or monzosy- results essentially identical to those presented here. tectonism and subsequent collapse and extension enite (Fuhrman et al., 1988). Using mineral norms with International Union of along the CCMZ (Bonamici et al., 2015). The follow- To the southeast of the complex is a coarse horn- Geological Sciences (IUGS) classification provides a ing work focuses on a portion of the Diana Complex, blende granite gneiss, termed the Lowville granite means by which to compare bulk-rock chemistry to including those outcrops studied by Bonamici et al. by Buddington (1939), which has been identified the X-ray diffraction–determined modes presented (2011, 2014, 2015). Because this research area fully as a member of the ca. 1100 Ma Hawkeye Gran- by Hargraves (1969). Buddington (1939) compared contains the CCMZ (Fig. 1), it allows integration of ite Gneiss suite (e.g., Chiarenzelli and McLelland, modal and normative analyses for these rocks and this work’s results with 40Ar/39Ar thermochronology, 1991; McLelland et al., 1996). However, the age of found them to be very comparable. Revealed by U-Pb titanite geochronology, metamorphic petro- this suite has been questioned (Walsh et al., 2016). the normative analysis presented in Figure 3A is logic investigations, and detailed structural analysis Also on the southeastern border of the complex is that the most mafic, pyroxene-bearing members data sets published for portions of the CCMZ and a body of anorthositic gabbro, plus a hornblende- of Diana Complex can be classified generally across the Adirondacks. and biotite-bearing aplitic granite gneiss that locally as monzodiorite to monzonite. The most felsic, appears to grade into the Diana Complex (Budding- hornblende-bearing rocks are generally quartz ton, 1939; Fig. 2). monzonite to granite. The aplitic granite gneiss nor- ■■ PETROLOGY AND U-Pb ZIRCON mative analysis plots with the felsic members of GEOCHRONOLOGY the Diana Complex. Lithological variation between Geochemistry these rocks is mostly caused by the indirect relation- The Diana Complex has long been identified as ship between plagioclase and quartz at a generally a deformed and metamorphosed granitoid mem- To help evaluate the relationships between the consistent alkali feldspar content (Hargraves, 1969). ber of the AMCG suite (McLelland et al., 1996). The granitoids in the study area, and investigate the The studied rocks all plot with other AMCG crustal northwestern contact between the Diana Complex possibility for a lithological discontinuity related melt rocks (MCG—mangerite- charnockite- granite)

Thin Section Traverse (Fig. 9) 4,889 471 CCMZ Boundary Magmatic Fabric 0 0.5 1 2 3 4 03-14 Foliation Strike and Dip 52 Kilometers 55 Lineation Trend H1 "east" Harrisville Diana Lineament Q1 68 Breccia Lineaments Metasediments Ductile Fault 45 05-01 High-strain Zone Boundary "west" High -strain Zone K3 CCMZ-C6

Pyroxene-bearing Diana Complex 69 45 49 34 03-15 Hornblende-bearing Diana Complex 79 80 59 Anorthositic Gabbro I3 03-21 55 4,882 Coarse Hornblende Granite Gneiss WP7 03-36 39 52 Aplitic Granite Gneiss 57 24 03-18 WP4 WP6 WP18 Metasediments 79

03-34 07-J 03-35 44 50CCMZ-J1 41 Kimball Mill

03-11 63 60 66

? CCMZ-812 53

4,875

70 Dutton Corners 464 478

Figure 2. Geologic map of the study area, Adirondacks of northern New York (USA). “West” and “east” refer to outcrops focused on by Bonamici et al. (2011, 2014, 2015). Map is based on Hargraves (1969), Isachsen and Fisher (1970), Geraghty et al. (1981), Wiener (1983), and this research. Site locations referred to in the text and figures are underlined. Geographic reference frame is UTM zone 18T, NAD27 datum, in thousands of meters. CCMZ—Carthage-Colton mylonite zone.

TABLE 1. BUL-ROC CHEMICAL ANALYSES within the Highlands on this and other diagrams diversity within the Diana Complex and neighboring Sample CCM-1 CCM-C6 CCM-812 considered (Seifert et al., 2010; Fig. 3). rocks of igneous origin, additional magmatic ages Lithology HSG PSG AGG More consistent with the syenitic terminology across the complex will help confirm the extent of SiO 65.18 57.44 69.03 2 commonly applied to the Diana Complex rocks, the the complex. TiO2 1.15 1.37 0.46

Al2O3 13.82 16.84 13.97 alkalis versus silica diagram shows that the com- Zircon was separated from a sample each of Fe2O3 6.76 7.86 4.67 plex plots in the high alkalis trachyte fields (Fig. 3B). the hornblende-bearing and the aplitic granite MnO 0.12 0.15 0.05 These rocks are also high in Fe relative to Mg and gneiss rocks through standard rock crushing and MgO 0.83 1.2 0.57 CaO 2.25 4.47 1.03 plot in the ferroan field (Fig. 3C). Harker plots magnetic and density separation procedures done

Na2O 3.79 4.58 3.56 (Fig. 3D) of these rocks define broad (e.g., Al2O3 at the University of Northern Colorado (Greeley, O 4.91 4.43 5.1 2 and Na2O versus SiO2) to tight (e.g., FeO and CaO Colorado, USA) (see Baird et al., 2014). Zircons P O 0.36 0.55 0.08 2 5 versus SiO ) linear trends typical of rocks related isolated from both samples were typically trans- LOI 0.50 0.7 1.2 2 Total 99.67 99.59 99.72 by fractional crystallization. This is also shown parent, light yellow to brown, euhedral, stubby to

Rb 48.5 131.9 192.50 on the linear trend on an AFM (Na2O + K2O, FeO*, elongate grains, ~200–400 microns in length, with Sr 638.7 278 105.30 MgO) diagram (Fig. 3E). Rare earth element (REE) the aplitic sample having somewhat smaller grains Ba 1534 795 443.00 Ta 0.4 1.1 1.40 plots (Fig. 3F) of the three rocks analyzed here all on average. Picked grains were mounted in epoxy, Nb 13.5 22.1 28.70 have a sloping light REE (La-Gd) profile, with the polished to expose cross-sections, and imaged with Pb 1.2 6.1 3.4 pyroxene-bearing sample (CCMZ-C6) having the cathodoluminescence (CL) to assess internal zoning Hf 19.2 21.3 18.80 shallowest slope and the aplitic sample (CCMZ-812) patterns. For both samples, nearly all grains contain r 887.8 899.6 741.20 Y 54.6 75.6 64.30 having the greatest slope. For all samples, the heavy oscillatory zoning consistent with magmatic crys- Sc 9 7 6 REEs (Tb-Lu) have flat profiles. The Eu anomaly is tallization. A few grains possess dark featureless Ni 0.7 1 0.5 slightly positive for the pyroxene-bearing sample, rims thick enough for analysis. n 117 73 110 Mo 1.5 2.1 5.1 moderately negative for the hornblende-bearing Uranium-lead (U-Pb) analysis of homogenous Co 7 7.7 2.6 rock (sample CCMZ-J1), and strongly negative for zones free of inclusions and cracks based on CL Sn 1 3 2.00 the aplitic rock. This pattern too is consistent with pattern was completed with the SHRIMP–reverse Sb 0.4 0.3 0.2 the rocks being related by fractional crystallization, geometry (SHRIMP-RG) instrument at Stanford Uni- 0.5 1.4 0.60 19 49 9.00 likely involving plagioclase. A similar pattern could versity (Stanford, California, USA) using standard Cu 3.3 7.8 2.9 be generated by variable amounts of partial melt- laboratory procedures (see Baird and Shrady, 2011, Th 1 9 19.40 ing, but this scenario is generally not considered and references therein). The spot size was ~21 μm, U 0.2 3 6.10 (Buddington, 1939; Hargraves, 1969). and data reduction, analysis, and presentation Ga 25.2 24.5 26.70 La 45.2 78.2 95.00 utilized programs based on Ludwig (2009, 2012). Ce 106 165.7 193.40 Table 2 contains the results of the SHRIMP-RG anal- Pr 14.37 21.12 22.44 U-Pb Zircon Geochronology ysis. All presented concordia diagrams display 1σ Nd 63.5 85.9 84.20 207 206 Sm 13.44 16.35 15.28 error ellipses, while Pb/ Pb age plots display 2σ Eu 4.4 2.97 1.77 The Diana Complex was previously dated via errors. Regression lines were calculated by York’s Gd 12.28 14.24 13.49 U-Pb zircon geochronology, and results provide algorithm (default method) and includes decay-con- Tb 1.97 2.31 2.13 magmatic crystallization ages of 1155 ± 4 Ma (ther- stant errors. Reported aggregated ages are reported Dy 10.49 13.53 11.68 Ho 2.1 2.75 2.33 mal ionization mass spectrometry [TIMS]; Grant at 95% confidence weighted by assigned errors; Er 6.05 8.24 6.86 et al., 1986), 1118 ± 3 Ma (single- and multi-grain also reported is the mean square of weighted devi- Tm 0.8 1.23 1.00 TIMS; Basu and Premo, 2001), and 1164 ± 11 Ma ates (MSWD). Yb 5.43 7.82 6.53 Lu 0.82 1.24 0.99 (sensitive high-resolution ion microprobe [SHRIMP]; Note: See Figure 2 for sample locations. Oxides are in Hamilton et al., 2004). The two results from TIMS weight ; elements are in ppm. UTM zone 18T NAD 27 work are questionable due to reverse discordance, Sample CCMZ-J1 easting and northing in meters: CCM-1 471073, 4869121; CCM-C6 471104, 4884166; CCM-812 471400, 4876202. excessive data scatter and Pb loss, and the identifi- As mapped by Hargraves (1969). HSGhornblende cation of multiple zircon growth domains. Therefore, Sample CCMZ-J1 is a typical sample of a uartz syenite gneiss, PSGpyroxene uartz syenite the SHRIMP-obtained age of Hamilton et al. (2004) strongly foliated and lineated hornblende-bearing gneiss, AGGaplite granite gneiss. All Fe reported as Fe2O3. from the H1 outcrop is likely the most robust, Diana Complex rock (Table 1). U-Pb zircon data LOIloss on ignition. but given the geographic extent and lithological have a modest spread, with most analyses ranging

20 3 CCMZ-C6 (Table 1) Q Diana Complex Powdered X-ray Modes (Hargraves, 1969) CCMZ-J1 (Table 1) CCMZ-812 (Table 1) q Highlands MCGs Norms/ Chemistry (Seifert et al., 2010) AM86-1 (Hamilton et al., 2004) 81 (Buddington, 1939) Diana Complex Norms 2 (Smyth & Buddington, 1926; C (Buddington, 1939) Buddington, 1939) Other Diana Complex qg 15 Analyses (Smyth & O (weight %)

g Buddington, 1926; (weight %) M

3 Buddington, 1939)

O 1 2 l A ag g gd t 10 0

6 Figure 3. Diana Complex geochemical data; qd/ 10 qmd/ qas qs qm qg/ symbols for all figure parts are found in A. qmg qa (A) Compilation of Diana Complex pow- 4 as s m md/mg d/g/a

O (weight %) dered X-ray mode data and normative a A P C

O (weight %) analysis of multiple bulk chemistry data e F 2 E sets shown on the International Union of FeO* Geological Sciences phaneritic rock classification diagram (after Le Maitre, 2002). 0 0 “MCGs” include the mangerite-charnockite- granite–type rocks near the Marcy massif Tholeiitic anorthosite complex in the Adirondack

5 O (weight %) Highlands (Fig. 1). Q—normalized quartz 2 a

N mode; A—normalized alkali feldspar mode; 1 P—normalized plagioclase feldspar mode; (weight %) 2

O q—quartzolite; qg—quartz-rich granitoid; i 4 T ag—alkali feldspar granite; g—granite; gd—granodiorite; t—tonalite; qas—quartz– Calc-Alkaline alkali feldspar syenite; qs—quartz syenite; qm—quartz monzonite; qmd/qmg—quartz 3 0 Alk MgO monzodiorite and quartz monzogabbro; qd/qg/qa – quartz diorite, quartz gabbro, 6 and quartz anorthosite; as—alkali feld-

O (weight %) 1

2 spar syenite; s—syenite; m—monzonite;

5 K 100 md/mg—monzodiorite and monzogabbro; d/g/a—diorite, gabbro, and anorthosite. 4

(weight %) (B) Alkalis versus silica diagram of Le Mai- 5 O 2 tre (2002). (C) Ferroan-magnesian diagram 3 P (Values are chondrite normalized) of Frost et al. (2001). FeO* is all iron as FeO. (D) Harker-type diagrams. (E) Alka- 2 0 Ce Nd Sm Gd Dy Er Yb lis-FeO*-MgO (AFM) diagram (Irvine and 50 60 SiO2 (weight %) 70 80 50 60 SiO (weight %) 70 80 2 La Pr Pm Eu Tb Ho Tm Lu Baragar, 1971). (F) Rare earth element plot normalized to chondrites (Sun and Mc- 12 Tephri- Trachyte 1.0 Donough, 1989). phonolite Trachydacite 10 P hono- Tephrite 0.9 (weight %) ferroan B O tr a R hyolite 2 a a 8 s y- K n c h d h a ac te

+ r i e y l T es - ti d 0.8 c n O*/(FeO*+MgO) O s a i 2 te e magnesian a

6 Trachy- F

N basalt B asaltic Dacite 0.7 B asalt Andesite 4 andesite 50 60 70 80 S iO (weight %) 45 50 55 60 65 70 75 2 S iO2 (weight %)

TABLE 2. SENSITIVE HIGH-RESOLUTION ION MICROPROBE–REVERSE GEOMETRY (SHRIMP-RG) DATA Spot U Th 232Th/238U 206Pb* Common 206Pb Disc. 204Pb corrected Error name (ppm) (ppm) (ppm) (%) (%) corr. Age (1σ error) 207Pb 1σ error 206Pb 1σ error 235U (%) 238U (%) 206Pb/238U 207Pb/206Pb

Sample CCMZ-J1 (UTM zone 18T, NAD27 easting and northing coordinates in meters: 471073, 4879121) J1-1.1 33 12 0.38 6 0.78 −25 1150 ± 20 935 ± 159 1.89 8.0 0.1954 1.9 0.244 J1-1.2 74 47 0.66 12 0.34 −2 1121 ± 25 1103 ± 60 2.00 3.9 0.1899 2.5 0.637 J1-1.3 339 91 0.28 52 0.02 +9 1061 ± 7 1158 ± 19 1.94 1.2 0.1789 0.7 0.596 J1-2.1 194 35 0.19 30 1.63 −14 1070 ± 16 944 ± 107 1.76 5.5 0.1805 1.6 0.299 J1-2.2 41 18 0.44 7 −0.18 −0 1223 ± 126 1218 ± 61 2.33 11.7 0.2088 11.3 0.965 J1-3.1 39 19 0.52 6 0.83 −14 1098 ± 16 974 ± 117 1.83 6.0 0.1857 1.6 0.273 J1-3.2 48 15 0.32 8 0.00 +1 1168 ± 16 1180 ± 48 2.17 2.9 0.1987 1.5 0.520 J1-3.3 85 24 0.30 13 0.00 +13 1082 ± 31 1233 ± 36 2.05 3.6 0.1827 3.1 0.865 J1-4.1 229 66 0.30 37 0.03 +2 1107 ± 8 1128 ± 42 2.00 2.3 0.1873 0.8 0.359 J1-5.1 373 64 0.18 57 0.10 +4 1058 ± 7 1095 ± 25 1.87 1.4 0.1784 0.7 0.495 J1-5.2 34 14 0.43 5 0.24 +8 1077 ± 18 1164 ± 79 1.97 4.4 0.1818 1.8 0.415 J1-6.1 40 16 0.41 6 0.00 +4 1111 ± 16 1150 ± 93 2.03 5.0 0.1881 1.6 0.324 J1-7.1 107 29 0.28 18 0.15 −2 1155 ± 11 1135 ± 40 2.10 2.3 0.1963 1.1 0.467 J1-7.2 134 50 0.39 22 0.06 +3 1117 ± 10 1148 ± 32 2.04 1.9 0.1892 1.0 0.526 J1-8.1 1020 48 0.05 172 0.00 +1 1152 ± 8 1163 ± 10 2.12 0.9 0.1957 0.7 0.803 Sample CCMZ-812 (UTM zone 18T, NAD27 easting and northing coordinates in meters: 471400, 4876202) 812-1.1 131 44 0.35 22 0.00 +6 1144 ± 10 1208 ± 27 2.15 1.7 0.1941 0.9 0.563 812-2.1 121 39 0.33 19 0.06 +11 1110 ± 10 1231 ± 33 2.11 1.9 0.1879 1.0 0.506 812-2.2 1109 87 0.08 178 0.02 +2 1102 ± 11 1120 ± 10 1.98 1.2 0.1865 1.1 0.911 812-3.1 173 77 0.46 28 0.06 +7 1106 ± 15 1182 ± 30 2.05 2.1 0.1872 1.5 0.689 812-3.2 36 12 0.34 6 0.21 −1 1168 ± 18 1159 ± 71 2.15 4.0 0.1986 1.7 0.434 812-3.3 1795 74 0.04 144 0.67 +30 576 ± 4 807 ± 24 0.85 1.4 0.0935 0.8 0.574 812-4.1 1668 56 0.03 138 0.65 +37 591 ± 13 916 ± 44 0.92 3.1 0.0960 2.2 0.724 812-4.2 273 79 0.30 46 −0.03 +2 1145 ± 8 1166 ± 21 2.11 1.3 0.1943 0.8 0.595 812-5.1 1616 95 0.06 265 0.00 −3 1126 ± 13 1092 ± 34 2.00 2.1 0.1908 1.2 0.593 812-5.2 97 39 0.41 16 0.00 +6 1128 ± 11 1199 ± 54 2.11 3.0 0.1911 1.1 0.368 812-6.1 226 66 0.30 38 0.00 +6 1153 ± 9 1215 ± 22 2.18 1.4 0.1958 0.8 0.592 812-6.2 2666 1167 0.45 271 0.40 +29 720 ± 7 993 ± 12 1.18 1.2 0.1182 1.0 0.852 812-7.1 126 148 1.21 17 0.38 +24 919 ± 8 1179 ± 50 1.68 2.7 0.1532 1.0 0.356 812-7.2 1615 60 0.04 238 0.03 +1 1020 ± 7 1027 ± 14 1.74 1.0 0.1714 0.8 0.758 812-8.1 108 37 0.35 18 0.15 −9 1157 ± 11 1073 ± 41 2.04 2.3 0.1967 1.0 0.452 812-9.1 1573 69 0.05 161 0.54 +22 726 ± 4 917 ± 18 1.14 1.0 0.1191 0.6 0.549 812-9.2 110 182 1.72 17 0.65 +10 1058 ± 10 1161 ± 57 1.93 3.1 0.1784 1.0 0.337 Note: See Figure 2 for sample locations. *—radiogenic; Disc. (%)—percent discordance relative to concordia = 100 * {[(207Pb/206Pb age) / (206Pb/238U age)] − 1}; corr.—correlation; UTM—Universal Transverse Mercator; NAD27—North American Datum of 1927.

from reversely discordant with 207Pb/206Pb ages in of 118 ± 620 Ma and 1161 ± 23 Ma, respectively. brighter oscillatory-zoned zircon (Fig. 4). Rare inher- the 900 Ma range, through to concordant or near- Because the lower intercept, within error, includes ited cores were not large enough to allow analysis. concordant 207Pb/206Pb ages of ca. 1150 Ma (Fig. 4A). the origin, the 207Pb/206Pb weighted mean age of A few analyses were rejected due to reverse discor- 1158 ± 16 Ma (MSWD = 0.28; Fig. 4A inset) from dance, large errors, or the analysis being an outlier. these nine analyses is interpreted to be the best Sample CCMZ-812 The resulting nine, tightly clustered 207Pb/206Pb age estimate of the igneous crystallization age of this analyses are concordant to near concordant and rock. Dark CL rims observed on some zircons do Sample CCMZ-812 is a typical sample of a mod- produce a chord with lower and upper intercepts not appear to possess an age different from that of erately foliated aplitic granite gneiss found south of

the rocks classically considered part of the Diana 1164 ± 79 1300 Complex (Hargraves, 1969; Table 1). Most U-Pb data cluster near concordia at ca. 1150 Ma, but a handful 1095 ± 25 0.22 of analyses are quite discordant or provide a much 1163 ± 10 younger 207Pb/206Pb age (Fig. 4B). Fitting a regres- 900 sion line through the seven analyses with the most 0.21 Mean = 1158±16 overlap in 207Pb/206Pb age produces a chord with MSWD = 0.28 1150 ± 93 700 lower and upper intercepts of 20 ± 550 Ma and 1182 ± 36 Ma. With the lower intercept of the cord 0.20 essentially zero, the 207Pb/206Pb weighted mean of 1150 U

8 1181 ± 25 Ma (MSWD = 0.30; Fig. 4B inset) from 3 2 / 0.19 these seven analyses is considered to be the best

P b 1128

6 estimate of the igneous crystallization age for this 0

2 ± 42 rock. No conclusive inherited cores were analyzed, 0.18 and dark CL rims are commonly highly discordant 1050 and do not appear to provide a coherent age, likely 1148 ± 32 due to radiation damage and significant Pb loss. 0.17 Intercepts at 1135 ± 40 118±620& 1161±23 Ma 1233 ± 36 MSWD = 0.30 Aggregate Age 0.16 1.5 1.7 1.9 2.1 2.3 207Pb/235U 1180 ± 48 Results for both rocks dated here and that of 0.21 Hamilton et al. (2004) are in good agreement col- 974 ± 117 lectively on a concordia diagram (Fig. 5), so all 1200 1150 1208 ± 27 analyses from the three rocks were used to calcu- 0.19 1100 late an aggregate age for the Diana Complex. The combined 27 analyses confirm 1164.3 ± 6.2 Ma as 1000 1050 916 ± 44 the age of the Diana Complex as a whole, including 90.0170 Mean = 1181±25 the aplitic granite gneiss. 800 MSWD = 0.30 950 1166 ± 21 1179 ± 50 0.15 Petrogenesis 850 1027 ± 14 U 8

3 The bulk chemistry and U-Pb zircon geochronol- 2 / 0.13 917 ± 18 750 ogy work presented here emphasizes the petrologic P b 6

0 1161 ± 57 relationship of the aplitic rock and the rocks classi- 2 cally identified as belonging to the Diana Complex. 0.11 807 ± 24 650 Intercepts at 100 µm 20±550& 1182±36 Ma All of these rocks are chemically similar to other MSWD = 0.36 granitoid members of the AMCG suite in the High- 1159 ± 71 0.09 lands (Fig. 3). 0.6 1.0 1.4 1.8 2.2 Hargraves (1969) and Buddington (1939) both 207 235 Pb/ U 1182 ± 30 envisioned that the Diana Complex formed from

Figure 4. (A) Concordia diagram for sample CCMZ-J1; inset is the 207Pb/206Pb ages for each analysis with the best estimate of a large, at least 6 km thick, stratified sheet-like the magmatic crystallization age (see text for details). Analysis symbols in gray were not used in any age calculation. (B) Con- intrusion whereby the pyroxene-bearing and 207 206 cordia diagram for sample CCMZ-812; inset is the Pb/ Pb ages for each analysis with the best estimate of the magmatic hornblende-bearing units are related by fractional crystallization age (see text for details). Analysis symbols in gray were not used in any age calculation. On the right are representative cathodoluminescence images with 207Pb/206Pb spot ages (in Ma); blue analysis locations are CCMZ-J1 zircons, red crystallization. With this model, Buddington (1939) analysis locations are CCMZ-812 zircons. MSWD—mean square of weighted deviates. believed the average Diana Complex composition

(composition C in Fig. 3) and the H1 outcrop (sam- characterized the transition between the coarse (Fig. 2; cf. Smyth and Buddington, 1926) and sug- ples 81, AM86-1 in Fig. 3) to be good estimates of hornblende granite and hornblende-bearing rocks gests that the body is even larger than originally the parent magma and surmised that perhaps a that are part of the complex as “imperceptible.” This estimated. larger quantity of granitic material formed from together with the field observations of this work Regardless of some of the finer details regard- the parent magma than was identified as Diana suggest that the coarse hornblende granite gneiss ing the evolution of parental magma(s), this work Complex at the time. Hargraves (1969, p. 353) (Lowville granite) too is part of the Diana Complex demonstrates that the Diana Complex crystallized at ca. 1164 Ma and includes the aplitic granitic gneiss and probably the coarse hornblende gran- 0.21 Pyroxene Diana Complex ite. This result confirms that no major lithological (Hamilton et al., 2004) discontinuity exists across the CCMZ within the Hornblende Diana Complex study area and that the Diana Complex indeed 0.20 (CCMZ-J1) shares affinities with both the Lowlands and the Aplitic Granite Highlands through intrusive, geochronologic, and (CCMZ-812) 1150 petrological relationships. The Diana Complex is a spatially extensive magmatic suite, and this likely 0.19 U

8 allowed the postulated kilometer- to tens of kilome- 3 2 / ters–scale displacements to occur across the CCMZ P b

6 without producing a lithological discontinuity. 0 2 0.18 1050 ■■ STRUCTURES

0.17 Magmatic Fabric Figure 5. (A) Concordia diagram with results from sample AM86-1 Original magmatic structure has largely been (Hamilton et al., 2004) and two 0.16 reoriented by subsequent deformation (e.g., Har- 1.5 1.7 1.9 2.1 2.3 samples presented here. (B) Com- 207Pb/235U pilation of 207Pb/206Pb ages from all graves, 1969), however in a few locations the aplitic analyses shown in A; the weighted granite gneiss does show layering that lacks any 1400 Mean = 1164.3±6.2 mean age of 1164.3 ± 6.2 Ma from all data in A is the best estimate of associated grain-shape or mineral-aggregate pre- MSWD = 0.88 magmatic crystallization of the en- ferred alignment (Fig. 6). This indicates that this tire Diana Complex. MSWD—mean fabric is preserved magmatic layering. Orientation square of weighted deviates. 1300 of such structures is essentially north-south, an orientation inconsistent with trends of deformational structures in the area.

1200 Regional Mylonitic Fabric Age (Ma)

1100 The regional mylonitic fabric (Hargraves, 1969; Geraghty et al., 1981; Baird and MacDonald, 2004; Fig. 2) in the Diana Complex is nearly ubiquitous Hornblende Pyroxene Diana Complex within the CCMZ and is penetrative in outcrops 1000 Diana Complex Aplitic Granite (Hamilton et al., 2004) (CCMZ-J1) (CCMZ-812) throughout the field area. Outside the study area, this fabric appears to exist as an axial planar folia-

Mean = 1164±11 Mean = 1158±16 Mean = 1181±25 tion to some Adirondack Lowlands folding (Wiener, 900 MSWD = 1.8 MSWD = 0.28 MSWD = 0.30 1983), but ductile deformation that postdates AMCG suite emplacement in most of the Lowlands has yet

The highest-strain regional mylonitic fabrics and associated microstructures can be mapped out as a band ~1–3 km wide within the central portion of the CCMZ (Fig. 2). This band was identified by A.F. Buddington (Smyth and Buddington, 1926, their map plate), and the northwestern boundary mapped here is nearly identical to that of Bud- dington, whereas the southwestern boundary is somewhat different owing to the more gradual transition moving out of the zone to the southwest. Thin sections taken from a traverse across the CCMZ show a progressive increase and then decrease in strain intensity as indicated by degree of recrystal- Figure 6. Magmatic folding of schlieren in the aplitic granite lization and strength of fabric (Fig. 9). Thin sections gneiss. The strike-dip symbol marked on a limb is nearly north- of the regional mylonitic foliation reveal two dis- south, with a vertical dip, which is the approximate orientation tinctly different sets of microstructures. The spatial of this fabric type. transition between the sets is in some places abrupt and is generally in close proximity to the transition from pyroxene- and hornblende-bearing units and to be conclusively demonstrated (Baird and Shrady, delineates the northwestern edge of the high-strain 2011). If it does exist, it may be restricted to areas zone (Figs. 2 and 9). This transition separates rocks bordering the CCMZ. To the southeast of the study with characteristics typical of Diana Complex rocks area in this paper, preliminary work suggests that adjacent to Lowlands paragneisses from those this fabric also at least weakly affects the rocks of typical of Highlands granitoids to the southeast. the Highlands for some distance to the southeast. For the purposes of this work, this transition is a Within the CCMZ study area, the regional mylo- convenient and reasonable feature taken as the nitic fabric does vary in intensity. At the CCMZ’s Lowlands-Highlands boundary. margins, this fabric is weak, whereas in the central Figure 10 summarizes the variation in percent CCMZ, this fabric is quite strong (Fig. 7). Though feldspar and quartz neoblasts, average quartz unrecrystallized feldspar porphyroclasts can be grain diameter, and average feldspar grain diam- identified, all of the regional mylonitic fabric dis- eter from >30 thin sections from across the CCMZ plays some amount of material that is finer-grained (Table 3). Plot distance was determined by the per- than the protolith owing to dynamic recrystalliza- pendicular distance from the Lowlands-Highlands tion and other processes (discussed below). Even boundary and the line that delineates the abrupt in nearly fully recrystallized rocks, the boundaries change in microstructural type (Fig. 2), with neg- of once-coherent feldspar grains are inferred from ative distances measured approximately to the the distribution of mafic minerals and quartz that northwest and positive distances being measured mantled the feldspar in the protolith. approximately to the southeast. Percent neoblasts Study area foliation typically dips ~30°–60° correlates well with the observed fabric strength to the northwest, except immediately along the (Figs. 2 and 9; Table 3). contact of the Diana Complex with the Lowlands The northwestern microstructures, termed Low- metasedimentary rocks and in the extreme south- lands microstructures, from northwest to southeast, Figure 7. Examples of Diana Complex regional mylonitic fabrics within the Carthage-Colton mylonite zone. All are subhori- ern part of the field area (Figs. 2 and 8). Mineral and show a profound increase in percent neoblasts such zontal surfaces with north approximately toward the upper stretching lineations are typically oriented ~40→350 that the rocks range from protomylonite to ultramy- right. (A) Moderately developed protomylonitic fabric in the and are best developed in areas with strong folia- lonite (Passchier and Trouw, 2005; Figs. 9A, 9B, 9C, northwestern part of the study area. (B) High-strain statically recrystallized mylonitic fabric in the central Carthage-Colton tion (Fig. 8). Locations without a strong lineation and 10). This change is accompanied by a mod- mylonite zone. (C) Weakly developed statically recrystallized commonly possess a well-developed S-C fabric. est increase in both quartz and feldspar neoblast mylonitic fabric in southeastern part of the study area.

Mesoscale Ductile Shear Zone Pole, n=69 Regional Lineation, n=12 Mesoscale Ductile Shear Zone Pole, n=76 Best-fit Axis for Ductile Shear Zones Regional Foliation Pole, n=34 Best-fit Axis for Ductile Shear Zones Best-fit Plane for Ductile Shear Zone Poles Best-fit Plane for Ductile Shear Zone Poles Lineation in Southern part of Study area Outcrop Regional Foliation Pole Foliation Pole in Southern part of Study area Least-deformed Hydrothermal Veins

Ductile Shear Zone Pole E Ductile Shear Zone Lineation, unknown kinematics Ductile Shear Zone Lineation (H1 outcrop in red, Fig. 2) Kinematics: shaded=away, unshaded=towards

S L

L Outcrop Lineation S Outcrop Foliation Great Circle

Mesoscale Ductile Shear Zone Pole

Figure 8. Equal-area projection stereonets. (A) Structural pattern for the regional mylonitic fabric (after Baird, 2006). (B) Geometry of mesoscale ductile shear zones of the study area, not including the H1 outcrop; pole of best-fit plane is 45→005 (after Baird, 2006). (C) Geometry of mesoscale ductile shear zones in the H1 outcrop; pole of best-fit plane is 64→339. Undeformed clinopyroxene-bearing hydrothermal vein orientations are also shown. (D) Ductile shear zone kinematics and lineation orientation (after Baird, 2006). For each lineation, the plane of the shear zone is traced by the line through the lineation symbol. (E) Ductile shear zone poles (n = 35) from locations that have definable regional lineation and foliation. All data have been rotated such that regional lineation (L) is horizontal east-west and regional foliation (S) is vertical east-west. In this view, regional shear is sinistral. Shear zone pole density is contoured with a 4% contour interval. Note the prominent conjugate set of shear zones, both oriented ~30° from the regional foliation, which bisects the acute angle of the set.

U U TG

5 mm U 5 mm QR 5 mm

5 mm E

Figure 9. Full thin-section scans of the regional mylonitic fabric across the Carthage-Colton mylonite zone. A–E are ordered in an approximately north-to-south traverse across the Diana Complex (Fig. 2). All are cross-polarized light; each thin-section orientation is parallel to lineation and perpendicular to foliation, with foliation approximately horizontal in the thin section with the view to the northeast or east depending on local fabric. Location references are shown in Figure 2. A–C are examples of rocks with Lowlands microstructure, while D–E are examples of Highlands microstructure; see text for detail. (A) H1 location; note the prominent undulose extinction (examples marked by U) and the Fe-Ti 5 mm oxide–filled tension gashes oriented upper right to lower left (examples marked by TG). (B) K3 location; protomylonitic S-C fabric with S surfaces oriented upper right to lower left and C surfaces oriented subhorizontally, indicating top-to-the-southeast transport. (C) WP7 location; well-developed mylonitic fabric with extensive recrystallization. (D) WP6 location; note the high strain and near-complete recrystallization with quartz forming prominent ribbons constructed of adjacent elongate grains (example marked by QR). Upper-central portion of the thin section displays a recrystallized σ-type porphyroclast with asymmetry indicating top-to-the-SSE transport. (E) WP18 location; note the similar microstructures compared to D, but of lower strain overall. S-C fabric is present with S surfaces oriented upper right to lower left and C surfaces oriented subhorizontally, indicating top-to-the-SSE transport.

100 grain size. Additional characteristics of Lowlands Ultramylonite 90 microstructure include feldspar porphyroclasts with strong undulose extinction that commonly 80 contain micro–tension gashes filled with Fe-Ti 70 oxides (Baird, 2001; Figs. 9A, 9B, 9C, and 11). Feld- 60 Mylonite

s (%) spar porphyroclasts also commonly have narrow t s a l 50 bands of neoblasts traversing them. Porphyroclast b o boundaries are always jagged and are surrounded

N e 40 by dynamically recrystallized quartz and feldspar 30 neoblasts in a polygonal mosaic, some of which 20 displays foam texture (Fig. 11). Individual neoblasts 10 have uniform extinction, and quartz and feldspar Protomylonite ←Lowlands Highlands→ 0 neoblasts can be distinguished from each other -5000 -4000 -3000 -2000 -1000 0 1000 2000 because quartz grains are distinctly larger in size Distance (m) (Fig. 10) and are commonly strung out into ribbons. Quartz, except in rare low-strain rocks (e.g., the H1 4500 ←Lowlands Highlands→ Figure 10. Variation in fabric characteristics across the Carthage-Colton outcrop), has completely recrystallized into equant 4000 mylonite zone. Hollow symbols are polygonal grains (foam texture). In the low-strain locations between the Diana linea- rocks, relatively large relict quartz grains display 3500 ment and the high-strain zone in the southwestern part of the study area undulose extinction and subgrains of similar size 3000 (Fig. 2). (A) Percent neoblasts, which to the surrounding quartz neoblasts, indicating that can be used as a proxy for strain. Fit- 2500 subgrain rotation is the dominant recrystallization ted line is a fourth-order polynomial mechanism (Fig. 11; Passchier and Trouw, 2005; 2000 merely to show the overall trend. Note the highest percent neoblasts Trouw et al., 2010). Rarely, and typically near the 1500 (strain) in the central portion of high-strain zone, quartz will show signs of static the Carthage-Colton mylonite zone, recrystallization by high-temperature grain-bound- 1000 correlating with the identified high- strain zone on the Highlands side of ary migration, as indicated by quartz ribbons being Average quartz neoblast grain size (µm) 500 the Highlands-Lowlands boundary partially constructed of wide single grains (Fig. 11D; (Fig. 2). (B) Average quartz neoblast Passchier and Trouw, 2005; Trouw et al., 2010). Many 0 grain size; error bars are one stan- -5000 -4000 -3000 -2000 -1000 0 1000 2000 dard deviation. (C) Average feldspar feldspar porphyroclasts, particularly those in the fur- Distance (m) neoblast grain size; error bars are thest northwest, lack subgrains with the suturing of one standard deviation. the grain boundaries equal in size to the surround- 700 ←Lowlands Highlands→ ing feldspar neoblasts, indicating recrystallization by low-temperature grain-boundary migration, also 600 called bulging (Fig. 11C; Passchier and Trouw, 2005; Trouw et al., 2010). In areas closer to the margin of 500 the high-strain zone in the core of the CCMZ, feld-

400 spars do show some subgrains of equal size to the surrounding feldspar neoblasts, indicating recrys- 300 tallization by subgrain rotation (Fig. 11E). The microstructures of the highest-strain zone, 200 extending to the southeastern edge of the Diana Complex, are distinctive and are termed Highlands 100

Average feldspar neoblast grain size (µm) microstructures (Figs. 9D, 9E, and 12). Except on

0 the southeasternmost edge of the Diana Complex, -5000 -4000 -3000 -2000 -1000 0 1000 2000 these rocks reflect more complete recrystallization Distance (m) and are classified as ultramylonite with rare relict

TABLE 3. SUMMARY OF MICROSTRUCTURAL DATA are only one grain thick wide (Fig. 12). This indicates Thin UTM- UTM- Thin section Feldspar Feldspar Feldspar uartz uartz uartz that extensive high-temperature grain boundary section Easting Northing neoblasts grain grain grains grain grain grains migration occurred during deformation and/or sub- (m) (m) (vol) average average measured average average measured sequent to deformation (Trouw et al., 2010). diameter diameter diameter diameter (m) standard (m) standard The characteristics that most clearly distinguish deviation deviation the two different microstructure types are that in (m) (m) the Lowlands type, the relict feldspar porphyro- L4B 472721 4879439 50 N/A N/A N/A N/A N/A N/A clasts are highly strained as indicated by: strong 10-3A 467928 4877677 65 112 63 15 365 160 6 undulose extinction, narrow bands of neoblasts 21-C 473762 4882660 80 115 50 14 298 253 12 traversing them, Fe-Ti oxide–filled micro–tension 07- 464955 4878984 15 39 16 17 87 33 9 gashes, and highly sutured grain boundaries. This S1 474576 4888297 30 15 8 15 49 23 14 is in contrast to the feldspar porphyroclasts in the Adk-Und 472531 4880600 95 236 87 12 945 591 3 1D1 473170 4886918 40 35 14 17 84 31 14 Highlands microstructure, which are more recrys- 1 471073 4879121 100 269 203 14 852 285 2 tallized, with the neoblasts showing evidence for Adk-15A 476760 4884044 97 158 70 26 889 835 3 grain-boundary area reduction. Other differences, Adk-34A 470916 4879521 98 290 318 10 N/A N/A N/A though not as universal, are the much larger grain E 468459 4878062 95 49 21 16 184 94 11 size for both feldspar and quartz in the Highlands Adk-16A 471297 4881513 50 54 12 13 88 29 13 microstructure (Fig. 10) and obvious evidence for C32 466897 4876229 60 57 19 15 157 63 13 I3L3 470842 4882659 70 33 14 15 59 25 15 high-temperature grain-boundary migration dis- O2B3 471141 4878957 70 381 141 11 623 272 12 played by quartz in the ribbons. F4U2 470744 4881623 60 32 9 13 45 22 13 M3C3 466466 4877013 90 88 30 14 173 107 14 N3C2 467263 4877810 90 80 41 15 345 179 13 Kinematic Indicators N7B2 467558 4879357 90 44 16 17 89 30 15 3I 470865 4884697 60 24 9 14 68 38 15 H1 473583 4887947 30 10 N/A N/A 51 19 15 Kinematic indicators associated with the C1Y3 466225 4877359 90 130 47 13 294 111 13 regional mylonitic fabric are found in multiple B6Y3 465929 4877719 99 59 27 17 121 79 12 locations. The Lowlands microstructures display B3C3 465845 4878156 80 44 19 16 367 237 14 S-C fabrics indicating top-to-the-SSE kinematics in C53 471101 4883833 65 37 14 17 134 48 15 a number of locations (Figs. 9B and 13A). In other O4A2 470639 4880966 96 76 27 15 170 80 15 localities, well-definedσ -type or more rarely δ-type O3C4 470692 4880277 96 134 62 27 459 279 25 C6L3 470998 4883406 65 26 5 16 62 21 16 kinematic indicators are observed. However, com- P7 471882 4881398 90 57 19 16 147 81 14 mon are weaker fabrics with poorly developed or P18 472309 4880547 98 374 162 13 2310 1660 8 no kinematics indicators. S-C fabrics, C-C′ relation- P6 472106 4880886 95 284 142 22 1569 1838 18 ships, and σ-type kinematic indicators are common P17 469405 4877526 99 191 52 12 476 188 11 in Highlands microstructure rocks, and these show Universal Transverse Mercator zone 18T, North American Datum of 1927. the same top-to-the-SSE transport determined from Note: N/Acould not be determined. the S-C fabrics in the northwestern part of the study area (Figs. 9D, 9E, 13B, and 13C).

porphyroclasts (Fig. 10). The porphyroclasts that This is indicated by grain-size variability (Fig. 10), do exist typically have straight extinction. The sur- straight grain boundaries forming near-120° triple Timing and Temperature Conditions rounding feldspar neoblasts commonly display a junctions, and larger grains displaying concave relatively coarse grain size with equant polygo- grain boundaries adjacent to smaller grains with Hornblende-plagioclase thermometry (Holland nal grains (foam texture) and show signs of static convex boundaries (Fig. 12; Passchier and Trouw, and Blundy, 1994) was used to assess conditions recrystallization by recovery, high-temperature 2005). Individual quartz grains have uniform extinc- of deformation. Analyses of recrystallized horn- grain-boundary migration leading to grain-bound- tion and always form ribbons constructed by a series blende and plagioclase were conducted with a ary area reduction, and likely grain-size coarsening. of elongate grains, such that the ribbons commonly JEOL 8900 electron microprobe at the University

of Minnesota–Twin Cities (Minneapolis, Minnesota, 500 µm 500 µm USA) using wavelength-dispersive spectrometry. The analysis procedure used appropriate natural mineral standards, 15 keV accelerating voltage, 20 nA beam current, and 2 µm beam diameter. Counting times on element X-ray peaks were 10–20 s, with 5–10 s counting time on upper and lower backgrounds. The volatile elements of Na and Cl were analyzed first and monitored to ensure no appreciable loss during analysis. Analyses used to calculate temperature were from neighboring or closely located hornblende and plagioclase grains (Fig. 14A). Chosen grains possessed a recrystallized polygonal texture, consistent with these grains E 500 µm reaching equilibrium during deformation and metamorphism. The hornblende and plagioclase compositions were similar for each phase for all analyses and were within the composition guide- lines prescribed for the thermometer (Holland and Blundy, 1994). Typical analyses from the regional mylonitic fabric are in Table 4 and summarized in Figure 14B. Results produce consistent deformation temperatures of 700–800 °C across the CCMZ at 7–8 kbar (Bohlen et al., 1985). Bonamici et al. (2014) identified the regional

100 µm mylonitic fabric as S1. No direct age constraint on this fabric exists other than that it must be younger 100 µm than ca. 1164 Ma. If this fabric is correlative with the fabric near Indian River Road in the southern CCMZ (Fig. 1B), it is older than an undeformed ca. 1040 Ma Lyon Mountain Granite Gneiss dike that crosscuts it there (Selleck et al., 2005; Johnson et al., 2005). Though its age cannot be tightly constrained, this fabric likely developed during Ottawan pro- grade metamorphism (Bonamici et al., 2014, 2015) at ~700 °C.

Mesoscale Ductile Shear Zones

Figure 11. Lowlands microstructures in thin section; all are cross-polarized light. Location references are shown in Figure 2. (A) Typical Throughout the study area, though more com- feldspar porphyroclast with undulose extinction, with bands of neoblasts traversing the grain (site I3). (B) Rare relict quartz grain mon in the pyroxene-bearing rocks of the Diana (upper center) displaying a relatively large size, undulose extinction, and subgrains. In the center are quartz neoblasts. Lower are a few feldspar porphyroclasts with undulose extinction and one Fe-Ti oxide micro–tension gash (upper right–lower left orienta- Complex, ~1–2-cm-wide (mesoscale) ultramy- tion); feldspar neoblasts are extremely fine grained and rim the feldspar porphyroclasts (site H1). (C) Mesoperthite porphyroclast lonite-cored ductile shear zones are found as displaying strong undulose extinction (left) with serrate grain boundary surrounded by feldspar neoblasts (site Q1). (D) Quartz individuals or in networks (Figs. 15A, 15B), such ribbon (center, subhorizontal) constructed of foam-texture grains, though one grain (right) is elongate indicating grain boundary migration (site WP7). (E) Feldspar porphyroclast (left and at center bottom) with subgrain boundaries (arrows) and finer-grained as in the H1 outcrop (Wiener, 1983; Lumino, 1986; feldspar neoblasts, some of which have straight grain boundaries resulting in foam texture (site WP7). Heyn, 1990; Cartwright et al., 1993; MacDonald,

500 µm

C 500 µm S

Figure 12. Highlands microstructures in thin section; both are cross-polarized light. Location references are shown in Figure 2. (A) Plagioclase feldspar porphyroclast (lower left center) with slight undulose extinction surrounded by relatively coarse- C grained foam-texture feldspar neoblasts (site WP18). (B) Quartz C’ ribbon constructed of adjacent elongate grains (center, sub horizontal). Also note the accessory mineral engulfed by quartz Figure 13. Example regional mylonitic fabric kinematic indicators (after Baird, 2006). Location references in the upper right central part of the photomicrograph (arrow) are shown in Figure 2. (A) S-C fabrics in a pyroxene-bearing Diana Complex rock in the northwestern (site WP18). part of the study area. Photograph is of a vertical face looking toward the ENE; kinematics is top-to- the-SSE. Approximately 5-mm-diameter ladybug for scale (arrow) (site 05-01). (B) Hornblende-bearing Diana Complex kinematic indicators from the southwestern part of the study area, including σ-type 1998; Baird and MacDonald, 2004). In many of these asymmetric porphyroclasts and C-C′ surfaces (horizontal and highlighted by rectangles, respectively). Rock face is perpendicular to the foliation and parallel to the lineation; view is toward the east and shows shear zones, the ultramylonite core is flanked by a top-to-the-south kinematics. Scale bar at lower right is 2 cm (site 03-35, sample CCMZ-J1). (C) σ-type transitional zone where the strain gradient results asymmetric porphyroclast showing top-to-the-southeast kinematics in a subvertical face. Edge of lens in a rotation of the surrounding fabric toward paral- cap for scale (200 m south of site WP7). lelism with the shear zone; this rotation can be used to indicate relative displacement. For some shear zones, the transitional zone is discolored compared et al., 2014). Ultramylonite cores of shear zones with the regional mylonitic fabric, suggesting that to lower-strain rock, suggesting that fluids altered have a grain size of ~10 μm in the northwestern the regional mylonitic foliation and ductile shear this rock (Cartwright et al., 1993). In particular, at part of the study area to ~100 μm in the southeast zones formed at the same time (Fig. 15C). the H1 outcrop, high-temperature fluids deposited of the field area. Due to limited exposures, termi- The orientations of a combined 145 meso clinopyroxene-rich veins that both are sheared by nations of mesoscale ductile shear zones are rarely scale ductile shear zones in the H1 outcrop and and crosscut the shear zones (Fig. 16; Bonamici observed, but those observed widen and merge in other locations in the study area are shown in

Figures 8C and 8B respectively. In either case, it is of the others, but the remaining 12 shear zones clear that shear zone poles lie generally along a display oblique-reverse kinematics. That is, irre- great circle, showing that an axis is approximately spective of shear zone orientation, the hanging parallel to the shear zones collectively; this axis is wall moved obliquely toward the south-southeast

K-spar approximately parallel to the regional lineation (cf. on average. Therefore, a top-to-the-SSE regional Fig. 8A). Heyn (1990) and MacDonald (1998) previ- shear, as is observed for the regional mylonitic fab- ously described such geometry. ric, could explain the kinematics of these 12 shear With the wall-rock fabric curving into the ultram- zones, including two shear zones studied in the ylonite core of the shear zone, it is reasonable to H1 outcrop.

pl conclude that the shear zones postdate the regional hbl mylonitic fabric. Though straight-walled shear zones are common (Fig. 15A) and clearly crosscut qtz Timing and Temperature Conditions the regional mylonitic fabric of the surrounding rock, some shear zone walls have undulations Hornblende-plagioclase thermometry (Hol- 50 µm that require shear zone wall deformation by shear land and Blundy, 1994) was also applied to the 15 associated with the regional mylonitic foliation. mesoscale ductile shear zones. Representative The undulation pattern is not found in the core microprobe analyses of recrystallized hornblende 14 of the shear zone nor on the opposite wall of the and plagioclase are presented in Table 5, with tem- 13 shear zone, and is best explained by synchronous perature calculations summarized in Figure 14. The 12 dynamic shear forming both the regional mylo- resulting conditions of 700–800 °C at 7–8 kbar are nitic fabric and the mesoscale ductile shear zone identical to those of the region mylonitic foliation. 11 (Fig. 15D). This is in agreement with the observa- Bonamici et al. (2014) identified the mesoscale 10 tions regarding shear zone terminations. ductile shear zones as S2. Detailed chemical, micro- 9 structural, and O, U, and Pb isotopic analysis of sphenes from within the shear zones indicate that 8 Ductile Kinematic Indicators these formed at ~700 °C and rapidly cooled to Shearing 7 ~500 °C ca. 1054–1047 Ma (Bonamici et al., 2011,

6 Thermometer A Due to near-complete recrystallization of shear 2014, 2015). zone ultramylonite cores, lineation in the core could

Pressure (kbar) 5 only be identified in thin sections cut parallel to 4 the fabric in the core. Lineation was identified Kinematic Model 3 by alignment of hornblende porphyroclast tails, opaque- or mafic-mineral train alignment, and zir- Bonamici et al. (2014) described a conjugate 2 Thermometer B con alignment (Fig. 17A). It is assumed that the set of shear zones within the H1 outcrop, which is 1 lineation approximates the slip vector within the also displayed in the data of Figure 8C, where the

0 ductile shear zone. regional mylonitic foliation bisects the two orien- 600 700 800 900 Shear zone kinematic interpretation was based tations. This pattern is also observable in the rest o Temperature ( C) on thin sections cut parallel to the lineation in the of the study area but only when the variation in the Figure 14. (A) Typical rock texture of grains used in horn- zone core and perpendicular to the plane of the regional mylonitic fabric orientation is accounted blende-plagioclase thermometry (backscattered electron image, shear zone. Criteria used to determine kinematics for. Figure 8E shows the mesoscale ductile shear after Baird, 2006). pl—plagioclase; hbl—hornblende; K-spar— included σ- and δ-type asymmetric porphyro- poles from locations that have a well-defined K-feldspar; qtz—quartz. (B) Results of hornblende-plagioclase thermometry from regional mylonitic fabric and mesoscale clasts and the rotation of wall-rock fabric through regional mylonitic foliation and lineation (seven ductile shear zones. Thermometer A corresponds to the reac- the shear zone transition zone (Figs. 17B, 17C). different locations). The shear zones have been tion edenite + 4 quartz = tremolite + albite; thermometer B Figure 8D shows the lineation orientation and rotated on the stereonet such that the regional corresponds to the reaction edenite + albite = richterite + anorthite. Bohlen et al. (1985) reported ~7–8 kbar pressures in the results of kinematic analysis of 14 mesoscale duc- lineation for that location is displayed as horizon- vicinity of the study area, which constrains ductile deformation tile shear zones. The sense of shear of one zone tal east-west and the foliation is vertical east-west. to ~700–800 °C (ductile shearing box). could not be determined, and one zone is opposite The pattern that results demonstrates that the

TABLE 4. REPRESENTATIE MINERAL ANALYSES AND TEMPERATURE CALCULATIONS FOR THE REGIONAL MYLONITIC FABRIC Lithology Pyroxene syenite gneiss Pyroxene syenite gneiss Hornblende uartz syenite gneiss Hornblende uartz syenite gneiss Site CCM-C6 03-13 03-35 03-15 Phase Amphibole Plagioclase Amphibole Plagioclase Amphibole Plagioclase Amphibole Plagioclase

SiO2 40.01 62.96 42.11 64.75 40.85 65.33 40.42 66.21

TiO2 1.32 N/A 0.85 N/A 1.75 N/A 1.27 N/A

Al2O3 11.76 23.14 9.68 23.79 9.76 21.36 10.50 21.82 FeO 22.37 0.43 18.40 0.20 26.95 0.37 23.31 0.34 MnO 0.30 N/A 0.41 N/A 0.49 N/A 0.83 N/A MgO 7.25 0.00 10.58 0.00 4.96 0.00 7.04 0.00 CaO 11.22 4.43 11.05 3.33 10.65 2.25 11.01 3.15

Na2O 1.48 9.21 1.53 7.86 2.00 10.75 1.68 9.87

2O 1.98 0.08 1.58 0.09 1.67 0.15 1.93 0.17 Cl 0.83 N/A 0.45 N/A 0.49 N/A 1.28 N/A Total 98.52 100.24 96.64 100.02 99.57 100.20 99.54 101.56 Cation Si 6.154 2.784 6.409 2.831 6.346 2.877 6.237 2.873 Al 2.131 1.206 1.737 1.226 1.788 1.109 1.910 1.116 Ti 0.153 N/A 0.098 N/A 0.205 N/A 0.147 N/A Fe2 2.000 0.016 1.187 0.007 2.807 0.013 2.245 0.012 Fe3 0.877 N/A 1.155 N/A 0.694 N/A 0.763 N/A Mn 0.039 N/A 0.053 N/A 0.064 N/A 0.109 N/A Mg 1.663 0.000 2.401 0.000 1.150 0.000 1.619 0.000 Ca 1.848 0.210 1.801 0.156 1.772 0.106 1.821 0.146 Na 0.440 0.790 0.451 0.666 0.602 0.918 0.503 0.830 0.389 0.004 0.306 0.005 0.331 0.009 0.381 0.009 Sum 15.695 5.010 15.598 4.891 15.758 5.032 15.735 4.988 Pressure Rxn A Rxn B Rxn A Rxn B Rxn A Rxn B Rxn A Rxn B (kbar) (C) (C) (C) (C) (C) (C) (C) (C) 0 829 709 824 725 873 700 882 721 5 788 727 766 735 804 708 817 731 10 746 746 709 745 736 717 752 741 15 705 765 652 754 668 725 687 750 Notes: See Figure 2 for sample locations. N/Anot analyzed; 0.00below detection limit. Reactions for temperature calculations: Rxn Aedenite 4 uartz tremolite albite; Rxn Bedenite albite richterite anorthite. As mapped by Hargraves (1969). Amphibole recalculations are done by the techniue outlined in Holland and Blundy (1994). Plagioclase is recalculated based on eight oxygens per formula unit.

mesoscale ductile shear zones in fact largely form rest of the study area because the regional foliation dominated bulk deformation (e.g., Choukroune a conjugate set, with the regional mylonitic foliation bisects the obtuse angle of the conjugate set at an and Gapais, 1983; Castro, 1986; Ring, 1999; Fossen ~30° from each shear zone set. Therefore, the girdle angle of ~55°–60°. Minor differences between the and Cavalcante, 2017). Figure 18B summarizes the pattern observed in Figure 8B is misleading regard- orientations of the shear zones at the H1 outcrop observed geometric and kinematic relationships ing the true nature of the shear zone geometry. The and across the field area are likely in part controlled between the coeval regional mylonitic fabric and conjugate nature of the shear zones is clear at the by the shear zones developing preferentially along mesoscale ductile shear zones. The observed H1 outcrop because the regional mylonitic fabric the generally steeply NW-SE–oriented clinopyrox- pattern is very similar to a synthetic one used by has a consistent orientation across the outcrop. ene-bearing veins common here (Fig. 8C). Hudleston (1999) to model the strain across shear However, the geometry between these structures Conjugate shear zone networks are common zone networks. However, natural examples of this is slightly different at the H1 outcrop than in the and can be formed by either pure- or simple-shear- pattern have not been described, but a similar

Figure 15. Images of mesoscale ductile shear zones. Lo- cation references are shown in Figure 2. (A) Slabbed rock containing a planar mesoscale ductile shear zone (after Baird and Hudleston, 2007). The transition zone of this shear zone is slightly discolored compared to less-deformed rock. Scale bar is 1 cm (site 03-14). (B) Network of intersecting mesoscale ductile shear zones, outlined in blue for clarity (after Baird, 2006). Strike-dip-lineation symbol provides regional mylonitic fabric attitude in this view; north is toward upper right. Shear zone kinematics shown are based on the ? rotation of fabric in the transition zone and are only relative. A core sample of a shear zone from this outcrop confirms that shear zone transport is oblique to outcrop surface; see text for details. Lens cap for scale is 52 mm diameter (site 03-18). (C) Mesoscale ductile shear zone merging with regional mylonitic fabric; boundaries of the shear zone are outlined (after Baird, 2006). Undulation in the outcrop surface causes some of the apparent variation in feature orientation. Pen is 14 cm in length (site 03-36). (D) Dotted line highlights the undulating margin of ultramylonite found within a mesoscale ductile shear zone (above line; after Baird, 2006). Outcrop surface is subhorizontal; weathering produces the mottled appearance in the upper right of the photo. Shear zone ultramylonite fabric orientation is indi- Regional Foliation cated by the strike-dip-lineation symbol. Mechanical pencil width is 8 mm (site 03-11).

Shear Zone

HTV Protomylonite

Figure 16. Features of the H1 outcrop (see Fig. 2 for location). (A) One of many hydrothermal veins comprising quartz, feldspar, and clinopyroxene with a myriad of accessory minerals, most notably sphene (see e.g., Bonamici et al., 2014). Field CPX book for scale is 19 cm long. (B) Mesoscale ductile shear zone (MSDSZ); such features commonly localize along hydrothermal veins, but here it is shown cross-cutting a hydrothermal vein (HTV). Note the shoe at the bottom for scale. (C) Ultramylonite mesoscale ductile shear zone cut by unsheared hydrothermal clinopyroxene (CPX).

MSDSZ Ultra- mylonite

Step 1 - pure shear trace foliation

Maximum Stretch 5 mm

500 μm Step 2 - simple shear Regional Shear Common Intersection Regional Lineation/ Shear Direction

~30o Regional Foliation 500 μm Shear Zone Lineation/ Shear Direction

Shear Zone Figure 17. Thin section images from mesoscale ductile shear zones. Location references are shown in Figure 2. (A) Four of the approximately dozen aligned zircons (outlined in yellow) found in the ultramylonite core of a mesoscale ductile shear zone thin section cut parallel to foliation. The zircons define a lineation interpreted to be the shear direction (site 03-11). Cross-polarized Figure 18. Block model displaying the two-step model that ex- light. (B) Plane-polarized light photomicrograph of shear zone transition zone fabric curving into the ultramylonite mesoscale plains the geometry and kinematics of the conjugate set of ductile shear zone core (dark subhorizontal band) with a dextral sense (site H1). (C) Plane-polarized light photomicrograph of a mesoscale ductile shear zones and the regional mylonitic fabric. dextral δ-type asymmetric porphyroclast found in a mesoscale ductile shear zone core (site Q1). View is toward the southwest. See text for details.

pattern is found in shear zone systems formed by and conjugate shear zones. During this initial stage, Hudleston, 2001; Baird and Hudleston, 2007). The pure shear (Choukroune and Gapais, 1983; Castro, strain accumulation must not have been great complex nature of solitary and interlinking shear 1986). Common to the studied and described sys- because beyond establishing the geometric rela- zones in bulk shear can account for the variation in tems is a symmetrical conjugate set of shear zones tionships, no other evidence of this deformation shear zone orientation, lineation orientation, and that intersect parallel to the regional lineation. The can be confirmed. kinematics compared to the ideal geometric model system studied here differs from the natural pure Then, in the second step, the deformation tran- presented in Figure 18B. shear–produced systems because the pure shear sitions to bulk simple shear (Fig. 18B), which more systems have opposing shear zone kinematics strongly develops the regional mylonitic fabric and and lineations perpendicular to the intersection kinematic indicators and overprints the kinemat- Possible Later Ductile Fabrics of the shear zones. To explain this discrepancy, a ics of the mesoscale ductile shear zones to what

two-step deformation model is used to explain the is observed. The shear zones are likely to remain Bonamici et al. (2014) identified an 3S fabric

studied system. active after the kinematic transition because they that folds the mesoscale ductile shear zones (S2) The first step in the model is the initial forma- are parallel to the bulk slip direction—a favorable at the outcrop scale. The one example found in tion of the regional mylonitic fabric and conjugate orientation for continued shear. With continued this work is shown in Figure 19. Stereographic shear zones from bulk pure shear (Fig. 18A). This simple shear, more shear zones likely develop analysis of this later fabric (Fig. 19C) shows that initial step established the geometric relationship with locally complex strain patterns where they its orientation is consistent with mesoscale ductile observed between the regional mylonitic fabric interlink (Hudleston, 1999; Bhattacharyya and shear zone orientations found throughout the area.

TABLE 5. REPRESENTATIE MINERAL ANALYSES AND TEMPERATURE CALCULATIONS FOR THE MESOSCALE DUCTILE SHEAR ONES Lithology Pyroxene syenite gneiss Pyroxene syenite gneiss Pyroxene syenite gneiss Hornblende uartz syenite gneiss Site 07- 03-14 03-21 03-15 Phase Amphibole Plagioclase Amphibole Plagioclase Amphibole Plagioclase Amphibole Plagioclase

SiO2 39.57 63.87 41.57 64.23 38.88 64.63 40.55 65.29

TiO2 1.36 N/A 1.30 N/A 1.36 N/A 1.20 N/A

Al2O3 11.67 22.86 10.44 22.44 11.81 22.37 10.31 22.08 FeO 23.65 0.25 20.42 0.23 26.16 0.21 22.84 0.56 MnO 0.25 N/A 0.54 N/A 0.63 N/A 0.86 N/A MgO 6.44 0.00 9.35 0.00 4.59 0.00 7.25 0.00 CaO 11.10 4.01 11.39 3.45 10.35 3.55 11.31 3.40

Na2O 1.72 9.51 1.62 9.74 1.64 9.80 1.61 9.83

2O 1.85 0.12 2.09 0.16 1.89 0.16 1.87 0.17 Cl 0.70 N/A 1.41 N/A 1.30 N/A 0.96 N/A Total 98.31 100.63 100.13 100.24 98.61 100.73 98.77 101.32 Cation Si 6.138 2.808 6.270 2.830 6.116 2.835 6.260 2.849 Al 2.133 1.185 1.856 1.165 2.189 1.156 1.876 1.135 Ti 0.159 N/A 0.147 N/A 0.161 N/A 0.140 N/A Fe2 2.338 0.009 1.821 0.009 2.548 0.008 2.170 0.020 Fe3 0.729 N/A 0.755 N/A 0.893 N/A 0.780 N/A Mn 0.033 N/A 0.069 N/A 0.084 N/A 0.113 N/A Mg 1.489 0.000 2.103 0.000 1.077 0.000 1.669 0.000 Ca 1.844 0.189 1.840 0.163 1.744 0.167 1.870 0.159 Na 0.517 0.810 0.474 0.832 0.499 0.833 0.482 0.831 0.366 0.007 0.402 0.009 0.379 0.009 0.369 0.010 Sum 15.746 5.008 15.737 5.008 15.689 5.008 15.729 5.004 Pressure Rxn A Rxn B Rxn A Rxn B Rxn A Rxn B Rxn A Rxn B (kbar) (C) (C) (C) (C) (C) (C) (C) (C) 0 864 712 884 726 818 707 884 717 5 817 730 817 734 779 727 818 726 10 770 747 749 743 741 747 751 734 15 723 765 682 751 703 768 685 743 Notes: See Figure 2 for sample locations. N/Anot analyzed; 0.00below detection limit. Reactions for temperature calculations: Rxn Aedenite 4 uartz tremolite albite; Rxn Bedenite albite richterite anorthite. As mapped by Hargraves (1969). Amphibole recalculations are done by the techniue outlined in Holland and Blundy (1994). Plagioclase is recalculated based on eight oxygens per formula unit.

Kinematic analysis of the zone based on the sense stereographically. Dating of sphene connected to by Lumino (1986), then this potentially suggests a of deflection of the mesoscale ductile shear zone such fabrics (Bonamici et al., 2015) provides an age rapid kinematic switch resulting in these scattered into the strong fabric suggests that shear sense for of ca. 1047 Ma, essentially identical to that of the late ductile fabrics. the strong fabric is dextral-oblique normal with top mesoscale ductile shear zones, and further supports to the north. However, a poorly developed σ-type the likelihood that a dynamic pure shear transi- kinematic indicator from the strong fabric suggest tioning to continuous top-to-the-SSE simple shear Ductile Fault sinistral-oblique reverse kinematics with top to the event can explain all deformational structures dis- south (Fig. 19D). Given the near parallelism of struc- cussed thus far. Alternatively, if the normal-sensed Hargraves (1969) identified a “ductile fault,” or tures, a small error in an orientation measurement kinematics on the late ductile fabrics holds true, more appropriately, a ductile shear zone (Fig. 2), in would provide erroneous kinematic determinations matching up with the high-strain zones investigated one outcrop and traced out the extent of this zone

to the southwest (see also Geraghty et al., 1981; Baird and MacDonald, 2004). The shear plane of the ductile fault is oriented ~070, 69 S, and based on the wrapping of the regional mylonitic fabric, has top- to-the-northwest kinematics (Figs. 20A and 20B). Timing of the ductile fault is unclear, but the structure does deform the regional mylonitic foliation and likely formed prior to deformation transitioning to a brittle style (see below).

Breccia Zones

Hargraves (1969) also identified a number of ~20–30-m-wide or wider breccia zones that strike generally NE-SW and commonly mark topo- 1” core graphic lineaments, the most prominent of which is named the Diana lineament (Fig. 2). Rocks along such zones are cataclasites and show evidence for veining which has also been brecciated (Figs. 20C, B 20D). In locations where brittle deformation is North not penetrative, the earlier ductile fabrics can be strong fabric SZ Lin lineation observed. These fabrics are finely laminated (cf. 1” core FA Hargraves, 1969) and have geometry and kine-

ultramylonite matics consistent with those of the regional strong fabric shear zone mylonitic fabric and are thus considered the same weaker penetrative fabric South SZ limb North (folded) SZ limb fabric (Fig. 20E). The breccia zones are the latest

merges with structures recognized, but exact timing of their for- penetrative

stronger penetrative fabric mation, or whether non-Grenvillian reactivation has occurred, is unclear. The orientation of such zones is also unclear, but given that the Diana lineament is closely related to the highest-strain fabric in the core of the CCMZ and near the transition between the pyroxene- to hornblende-bearing lithologies, at least this breccia zone is likely subparallel to the regional mylonitic foliation. Localized brittle shearing can be found throughout the length of the CCMZ, and in the Dana Hill metagabbro (Fig. 2), brecciation developed under greenschist facies conditions at ca. 1000–930 Ma (Johnson et al., 2004).

Figure 19. Meso-scale ductile shear zone deformed by a later fabric. (A,B) Annotated outcrop sketch (A) and photo (B). The orientation of the weak fabric could not be determined with confidence (site 07-J; see Fig. 2 for location). (C) Stereonet of structures in A and B. SZ—shear zone; Lin—lineation; FA—fold axis of fold in shear zone. Given the geometry of foliation and lineation of the Chronology of Deformation and Related strong fabric and the rotation sense of the fold into the strong fabric, kinematics on the strong fabric are normal sensed. (D) Core Events cut parallel to lineation and perpendicular to foliation shows weakly developed top-to-the-south reverse-sensed (dextral) kinematics. Height of image is about 2 cm. Figure 21 shows a schematic vertical cross-section through the CCMZ with the major features

depicted. In the text below, numbers in parentheses are keyed to features in the figure. Earliest deformational structures include the penetrative regional mylonitic foliation, which typically manifests as an S-C fabric within the more mafic pyroxene-bearing rocks along the northwest of the zone, to higher-strain ribbon gneisses with various kinematic indicators in hornblende granitic rocks on the southwestern side of the zone (0 and 1). Gen- erally cross-cutting the regional fabric, with local evidence for coeval shearing, are the mesoscale ductile shear zones (2), which are best developed in the pyroxene-bearing rocks. Fluid infiltration, which precipitated localized pyroxene-bearing hydrothermal veins (2), was synchronous with mesoscale ductile shear zone development. Localized later shear fabrics (3) may in some places deform the Folded Lineation mesoscale ductile shear zones. Kinematics are Folded Lineation Best-fit Plane Folded Foliation Pole unclear on such zones, but all examples have geom- Folded Foliation Poles Best-fit Plane etry consistent with that of the mesoscale ductile Dutile Fault Plane shear zones, so they may be part of this structural Calculated Slip Direction suite. The work presented here, augmented and supported by geochronology, demonstrates that the regional mylonitic fabric, mesoscale ductile E 1 cm shear zones, pyroxene-bearing hydrothermal veins, and possibly later shearing of the mesoscale ductile shear zones, were all driven by top-to-the-SSE oblique-sinistral reverse shearing at 1054–1047 Ma (Bonamici et al., 2015) during rapid cooling from ~700 °C to ~500 °C, likely over just a few million years (cf. Figs. 14B and 18). This was followed by development of the ductile fault (4) under a different deformation paradigm from above, because this structure could not have been formed by the same shear kinematics as for the regional mylonitic foliation and mesoscale ductile shear zones. The latest structures are the breccia zones (5), with presum- ably all examples formed at the same time. Timing and kinematics on the breccia zones are unknown Figure 20. Photos and analysis of other structures in the study area. Location references are shown in Figure 2. (A) Looking ap- here, hence the query symbol used in Figure 21. proximately east (mirror image from reality to make the relative view and kinematics the same as all other figures shown in this report) at one of the shear zones that compose the ductile fault in a road outcrop; solid curved red line traces regional mylonitic foliation that can be seen as alkali feldspar–rich layers above the red line; dashed red line traces ductile fault shear plane. Brunton compass for scale (arrow) (site 03-34). (B) Stereonet construction of slip direction of the ductile fault based on shear plane and ■■ DISCUSSION folded lineation orientation (Ramsay, 1967). Slip direction is calculated to be 56→101. Arrows indicate rotation direction from typical regional mylonitic fabric orientations. Data are from this work and Hargraves (1969). (C) Breccia along the Diana lineament (between sites WP6 and WP18). (D) Brecciated quartz vein along the Diana lineament (520 m north of site 03-34). (E) Billet of Bulk-rock chemistry, U-Pb zircon geochronol- high-ductile-strain fabrics found along the Diana lineament; view to the northeast; one top-to-the-SSE δ-type kinematic indicator ogy, and field relationships demonstrate that the is highlighted (site WP4). Diana Complex traverses beyond both margins of

pyroxene-bearing or hornblende-bearing (Fig. 2), and microstructures is clear and suggests a funda- West/Northwest 0-lithology cpx-bearing mental difference between these rocks. Adirondack S-C Fabric Figure 21. Schematic vertical cross-sec- Lowlands tion, view toward the northeast, through Clearly the modal proportions of pyroxene and high-strain the Carthage-Colton mylonite zone (CCMZ) hornblende are not high enough to have much of “Hanging Wall” fabric and Diana Complex highlighting structures, an influence on the whole-rock rheology, so the their approximate current-day orientation, 1-Regional Fabric hbl-bearing and kinematics. Features are numbered contrasting microstructure is likely controlled by and color coded to match expanded de- rock differences other than accessory mineralogy. scription in text: 0—present-day general Charnockitic and related crustal-derived rocks form 2-MSDSZ lithology distribution from clinopyroxene and Fluid (cpx)–bearing to hornblende (hbl)–bearing from dry melts (Emslie et al., 1994), thus leading Diana Complex; 1—regional mylonitic fabric to accessory magmatic pyroxene (Hamilton et al., formed by oblique-sinistral shear and re- cpx- 2004) and predominately a single (now exsolved) sulting in S-C fabrics and high-strain fabrics; veins ? feldspar. However, the hornblende-bearing rocks 3-localized normal- or 2—mesoscale ductile shear zones (MSDSZ) reverse-sensed forming by the same shear as the regional require additional explanation. One explanation is 4-Ductile Fault shearing? mylonitic fabric and synchronous with fluid that the melt that produced the hornblende-bearing deposition of cpx-bearing veins; 3—poten- rocks was hydrous, resulting in primary hornblende ? tially some later shearing of the MSDSZ; C C M Z 4—ductile fault formed by reverse-sensed (Buddington, 1939). Alternatively, the hornblende shearing; 5—breccia zones of unknown ori- could have been a result of selective hydration of a East/Southeast entation cutting all previous structures. See text for detail. pyroxene-bearing rock prior to or during deforma- Adirondack tion, such that during deformation all pyroxene in Highlands 5-Breccia Zones the hydrated rocks was lost in favor of hornblende. “Footwall” In either case, the hornblende-bearing rocks would have been initially more hydrous, or became so, and the water content could explain the microstructural the CCMZ, is expansive, and has affinities to both across the CCMZ and cannot explain the differing difference. However, the hornblende-bearing rocks the Adirondack Lowlands and Highlands. Therefore, microstructures of the Lowlands and Highlands are higher in quartz and poorer in plagioclase com- the inferred kilometer to tens of kilometers of CCMZ microstructure types. pared to the pyroxene-bearing rocks (Hargraves, shearing after ca. 1164 Ma complex intrusion did Many laboratory experiments have investi- 1969; Fig. 3A). This mineralogical difference, either in not result in a lithological discontinuity. The com- gated deformation mechanisms and the resulting tandem with or independently of the water content, plex records a penetrative top-to-the-SSE, rapidly microstructures of deformed pure quartz rocks (e.g., could have influenced rheology, microstructures, cooling, high-temperature shearing event at ca. Stipp et al., 2006, and references therein). In general, and structures formed in each rock type as well. 1054–1047 Ma (Bonamici et al., 2011, 2014, 2015). No mylonitic rocks with fine-grained neoblasts, such The tectonic significance of the later ductile evidence exists for an earlier, late-Shawinigan-aged as those found in the Lowlands microstructures, fault and breccia zones clearly indicates significant deformation of the Diana Complex as previously could be produced not only by lower-temperature changes in the deformation style following the pen- postulated. ductile deformation, but also by some combination etrative ductile event and must be accounted for Though the microstructures studied here may of fast strain rates, high differential stress, and dry in any model. Similarly, any model for the CCMZ be interpreted to have formed under different tem- deformation conditions. Alternatively, the High- must successfully incorporate differing kinematics perature conditions, with higher temperatures to lands microstructures could have been produced along the CCMZ from those presented here (e.g., the southeast, the temperature estimate for defor- by some combination of higher temperatures, Johnson et al., 2004), the cooling histories of the mation across the zone presented here is consistent metamorphism outlasting deformation, low strain Highlands and Lowlands (Streepey et al., 2001; Dahl with peak temperature estimates of ~700 °C by rates, low differential stress, and wet deformation et al., 2004), and the steepness and complex sin- Bonamici et al. (2011, 2014, 2015) along the north- conditions. Interpretation of microstructures is uous (folded?) map pattern of the northern half of western edge of the zone. Uniform temperature further complicated when considering polyminer- the zone (Fig. 1B). Below, two tectonic models are estimates of the deformation all come from recrys- alic rocks or varying mineral modes, including the considered to best explain the collective knowledge tallized grains within the regional mylonitic foliation potential for varying mineral compositions (e.g., regarding the Diana Complex and its deformation and mesoscale ductile shear zones, suggesting that Herwegh et al., 2011). However, the close connec- history associated with the CCMZ and surround- deformation temperature is at least broadly similar tion between lithology, here summarized as either ing terranes.

Model 1 along the CCMZ (Dahl et al., 2004). This thermal ca. 1045–1037 Ma (Selleck et al., 2005; Fig. 22C) and structure may have been produced by a growing the East Adirondack shear zone at ca. 1050–1025 Ma Model 1 considers the CCMZ as an Ottawan- Ottawan orogen toward the northwest from the (Wong et al., 2011), thus forming a symmetrical met- aged top-to-the-SSE oblique thrust that was later locus of Amazonia collision to the southeast of the amorphic core complex. Within the study area, the reactivated with one or more normal-sensed shear- study area. Oblique CCMZ thrusting occurred when normal-sensed late shear fabrics and/or the brec- ing events that exhumed the Adirondack Highlands the Diana Complex rocks reached ~700 °C tempera- cia zones may record this event. Such zones may (Fig. 22). The initial event of this model is the ca. ture, with higher temperatures to the southeast correlate with the greenschist facies brecciation 1050 Ma creation of the CCMZ by pure shear tran- in the Highlands and lower temperatures to the found in the Dana Hill metagabbro that may have sitioning to oblique thrusting (Fig. 18) at peak northwest in the Lowlands. As the Lowlands moved formed ca. 1000–930 Ma during cooling through metamorphic conditions (~700 °C; Bonamici et al., to higher structural levels, this may have largely ~300 °C and final juxtaposition of the two terranes 2011; this work) during collision of Amazonia into protected it from the thermal, penetrative ductile (Streepey et al., 2001; Dahl et al., 2004). Laurentia (Fig. 22A). This thrusting included kilome- deformation and magmatic effects of this orogeny. This model is largely consistent with all data ters or more of displacement, possibly along a listric Regardless of the isotherm details, continued and tectonic models that require crustal thickening CCMZ that tilted the Lowlands down to the north- thrusting led to localized back-thrusting as recorded preceding tectonic collapse (see summary in Rivers west, leading to inclined isotherms (Dahl et al., 2004). by the ductile fault in the study area and steep- [2011]), though it does not easily explain the ductile In this model, the Lowlands largely escaped Ottawan ening and/or folding of the northern half of the dextral strike-slip motion recorded in the Dana Hill tectonism because it was at a shallower crustal level CCMZ (Fig. 22B). The Lowlands then began to cool metagabbro at similar peak conditions (710–770 °C) where thermal, magmatic, and penetrative ductile as isotherms relaxed. Thrusting of the Adirondack and timing (1050–1030 Ma; Johnson et al., 2004, deformation associated with this orogeny were lim- Lowlands over the Adirondack Highlands is consis- 2005) to the ductile deformation studied here. A ited. If this were the case, then later normal-sensed tent with rapid cooling of the Lowlands side of the possible explanation is that this dextral motion may motion would have been very significant, suggest- zone (Bonamici et al., 2011; 2014; 2015) against the have been of only local importance for the Dana Hill ing that structures producing this motion would be cooler, shallower crustal portions of the Highlands. metagabbro deformation, but not to the CCMZ as more prominent, which is not observed. This thrusting was closely followed by Lyon a whole. Further, normal-sensed ductile deforma- Alternatively, isotherms may have been inclined Mountain Granite Gneiss intrusion and exhumation during this interval is evident in the Dana Hill across the Adirondacks prior to oblique thrusting tion of the Adirondack Highlands along the CCMZ at body, but may be completely lacking in the Diana Complex, or at minimum, is very poorly developed. This is problematic if the CCMZ facilitated Adiron- dack Highlands exhumation, as this should be a Far-field o C o C o C N ~500 ~700 ~500 stress penetrative deformation event. Possible solutions to this conundrum include that such extensional Toward back- folding structures (1) were recorded outside of the study Away thrust area to the northwest or southeast, (2) are poorly exposed within the study area, or (3) have been CCMZ Lowlands Highlands eradicated by brecciation. ca. 1060-1050 Ma - Ductile ca. 1050-1040 Ma - Significant thrusting and formation of thrusting leads to folding and the Carthage-Colton Mylonite back-thrusting along CCMZ Figure 22. Tectonic model 1 explaining Model 2 Zone (CCMZ) the history of the Carthage-Colton my- Lyon Mountain lonite zone. See text for added detail. Granite Model 2 considers the CCMZ as originating as a subhorizontal mid-crustal mylonite zone associated with the ca. 1060–1050 Ma tectonic collapse of the now-built Ottawan orogen formed during Ama- zonia-Laurentia collision (Fig. 23A). The Lowlands 300oC would be structurally higher than the Highlands in ca. 1040 Ma - Ductile normal- ca. 945 Ma - Low-grade brecciation a region that received little thermal, magmatic, and sensed motion, observed at along the CCMZ and final juxtaposition some locations along the CCMZ, of the Lowlands and Highlands. penetrative ductile effect due to the Ottawan orog- coeval with Lyon Mountain Granite. eny, while the Highlands received extensive Ottawan

local variation in southeast, became arched with exhumation of the shear sense and displacement Adirondack Highlands. During arching, once-horizontal isotherms in the Adirondack Lowlands Brittle-Ductile Transition became tilted (Dahl et al., 2004), but the southern LL CCMZ preserved the kinematics and structures CCMZ Mid-crustal Mylonite Zone imprinted when it was subhorizontal (including HL Diana Complex Zone of distributed deformation the regional mylonitic foliation and mesoscale ductile shear zones). The Dana Hill metagabbro in the ca. 1060-1050 Ma - Tectonic collapse initiates. central CCMZ preserved a different kinematics as dictated by a different local architecture of exten- Folding? Detachment Normal-sensed Figure 23. Tectonic model 2 explaining sional structures. The top-to-the-northwest ductile Reactivation? the history of the Carthage-Colton my- fault was produced by vertical simple shear that 500oC EASZ lonite zone (CCMZ), based on figures in occurred during doming of the mid-crustal mylonite o LL Cooper et al. (2010) and Platt et al. (2014). 700 C HL zone; such structures are also found in the Brenner CCMZ Figure is highly schematic and is of an Ductile approximately northwest-southeast line of the Alps (Axen et al., 1995). Overprinting of Toward Relict Away Fault Lyon Mountain Granite cross-section through the Adirondack Kinematics structures with opposing shear-sensed structures Lowlands and Highlands. See text for are also found in the Snake Range décollement ca. 1050-1040 Ma - CCMZ domed over Highlands as core complex added detail. LL—Adirondack Lowlands; (western United States) and are also interpreted to develops, formation of ductile fault, perhaps folding of CCMZ, and HL—Adirondack Highlands; EASZ—East Adirondack shear zone. have formed as the mylonite zone became domed local normal-sensed overprinting, Lyon Mountain Granite intrusion. during exhumation (Cooper et al., 2010). Modeling studies of metamorphic core complex formation (Lavier et al., 1999; Platt et al., 2014) demonstrate that the mid-crustal mylonite zone can be folded and Approximate erosion level rotated to steep orientation during doming and may 300oC LL HL EASZ CCMZ explain such characteristics found in the northern brecciation half of the CCMZ. However, an alternative explanation for the locally folded character of the CCMZ ca. 945 Ma - Low-grade brecciation along the CCMZ and final is that metamorphic core complexes or ductilely juxtaposition of the Lowlands and Highlands. extended crust commonly exhibits corrugations or cross-folds (e.g., Fossen, 2016; Brown et al., 2016; Schwerdtner et al., 2016), produced by transtension tectonism. The CCMZ would develop between the shear possibly due to crustal thinning, but this gave (Fossen et al., 2013). Lyon Mountain Granite Gneiss Lowlands and the Highlands as the subhorizontal way to simple shear–dominated deformation as the intruded the Adirondack Highlands too during sole of a complex series of extensional shear zones extensional shear zone architecture established and exhumation and locally along the CCMZ (ca. 1045– of various geometries at the initial stages of tectonic drove the deformation along the mid-crustal mylon- 1037 Ma). Accompanying intrusion, early-formed collapse. Such a subhorizontal mid-crustal mylonite ite zone. As the mid-crustal mylonite zone formed at CCMZ ductile structures were locally overprinted zone has been described by, e.g., Cooper et al. (2010) ca. 1054–1047 Ma at ~700 °C, the orogen as a whole with extensional ductile structures (e.g., Johnson and Platt et al. (2015) and, depending on the geom- began to cool, which continued during the duration et al., 2005; Selleck et al., 2005). During exhumation, etry of the extensional shear zones, would have of Ottawan extension, consistent with the data of the East Adirondack shear zone (ca. 1050–1025 Ma; variable kinematics and displacement along the Bonamici et al. (2011, 2014, 2015). Within 10–20 m.y. Wong et al., 2011) acted as the ductile extension of structure. The discrepancy between the essentially of formation of the mid-crustal mylonite zone, one the master detachment. Lastly, at ca. 945 Ma, low- uniform top-to-the-SSE kinematics studied here and detachment dominated extension and the Adiron- grade brecciation occurred along the CCMZ as the the dextral transcurrent kinematics of similar timing dack Highlands began doming in isostatic response ~300 °C isotherm flattened out and soon migrated and conditions found in the Dana Hill metagabbro and produced an asymmetric metamorphic core through the CCMZ and Highlands (Streepey et al., (Fig. 1; Johnson et al., 2004, 2005) are therefore complex (Fig. 23B). The mid-crustal mylonite zone, 2001; Dahl et al., 2004; Johnson et al., 2004; Fig 23C). expected for such structures. In the earliest stages including the sections referred as the CCMZ in the This model is largely consistent with all data sets, of CCMZ deformation, deformation was by pure northwest and the East Adirondack shear zone in the as is model 1. However, model 2 additionally can

account for the differing kinematics found between than for other locations studied in detail b. Model 2 depicts the CCMZ as initially the Diana Complex structures studied here and along the CCMZ; this emphasizes the need forming as a subhorizontal mid-crustal the dextral transcurrent structures of the Dana Hill to study multiple segments of crustal-scale mylonite zone during tectonic collapse. metagabbro due to local variation in extensional shear zones in order to fully understand their The collapse led to exhumation of the structures before doming. Further, it suggests that tectonic history and importance. Highlands and doming of the mid-crustal the shear zones found throughout the Adirondack 2. The Diana Complex is a member of the mylonite zone over the Highlands to Lowlands (Fig. 1B) may have had an extensional AMCG suite, spans the CCMZ, and crystal- form the CCMZ (in the northwest) and or transcurrent history during Ottawan collapse lized at 1164.3 ± 6.2 Ma. Original estimates of the East Adirondack shear zone (in the and may sole into the CCMZ at depth below the complex minimum thickness of 6 km likely southeast), resulting in an asymmetri- Lowlands. This model also suggests that a lack of underestimate its extent, because this work cal metamorphic core complex. During normal-sensed structures found in the study area demonstrates the inclusion of, at minimum, doming, the CCMZ was overprinted with may be expected because such structures are not the aplitic granite gneiss to the complex. additional structures. The top-to-the-SSE required to form during Highlands exhumation. Therefore, the inferred kilometer to tens of preserved kinematic indicators in the The overall top-to-the-SSE kinematics is consistent kilometers of offsets that occurred across southern CCMZ originated during the with the warped ca. 1070–1060 Ma Marcy Massif the CCMZ did not result in a lithological dis- subhorizontal shearing before doming. detachment zone described by Regan et al. (2019) continuity across the CCMZ. Model 2 is the preferred model because and suggests that this deformation style may be 3. The Diana Complex within the CCMZ con- it best integrates all data. pervasive throughout the Highlands. tains a northwest-dipping penetrative However, a potential drawback of model 2 is regional mylonitic foliation that bisects that if the Highlands were at least partially migma- a conjugate set of mesoscale ductile ACKNOWLEDGEMENTS titic at the time of exhumation (Bickford et al., 2008), shear zones. These structures initiated Portions of this research began under the advisement of William it is expected that a dome structure, potentially with in a pure-shear event that transitioned to D. MacDonald at the State University of New York at Binghamton in 1999 as part of a Master’s thesis. Starting in 2003, research sub-domes (e.g., Whitney et al., 2013), should be oblique-sinistral, top-to-the-SSE shearing continued during Ph.D. studies under the advisement of Peter J. evident in the Highlands, as is seen in the Morin and that mostly developed the structures. Hudleston at the University of Minnesota–Twin Cities. Chloë Mékinac-Taureau domes to the north (Dufréchou, 4. Strain associated with the regional mylonitic Bonamici and Giovanni Musumeci provided exceedingly helpful reviews of this manuscript that led to the clarification of much of 2017). To date, no domes or sub-domes that could foliation is greatest in the central CCMZ; this the data and ideas presented. Additionally, during the approx- be connected to exhumation have been described zone of high strain is associated with a change imate 20 years of work on this project, beyond my graduate in the Highlands. Current mapping depicts the in microstructure. This change is a marker advisors, numerous other individuals provided significant help and feedback including Annia Fayon, Cathy Shrady, Fawna Kor- Highlands possessing an arcing northeast-south- that can be used as the boundary between honen, Eric Johnson, Tim Grover, Toby Rivers, and a number of west to east-west to northwest-southeast structural the Adirondack Lowlands and Highlands. anonymous reviewers. Editorial handling of the manuscript by grain from west to east (Fig 1B; McLelland and 5. Ductile deformation of the Diana Complex Gina Harlow, Andrea Hampel, and Francesco Mazzarini is greatly Isachsen, 1980). With more detailed mapping in occurred at ca. 1054–1047 Ma (Bonamici et al., appreciated. This paper is dedicated to the memory of Erik J. Kent, a fine geologist and friend who assisted with fieldwork the Highlands, structures consistent with the asym- 2015) at ~700 °C. Later structures include a and provided feedback on early versions of this manuscript. metric metamorphic core complex model (model ductile fault and breccia zones. Conditions Research funding included a GSA Graduate Student Research 2) may be identified (see Regan et al., 2019). How- and kinematics of brecciation are less clear, Grant (7327-03), a U.S. Department of Education Graduate Assis- tance in Areas of National Needs Fellowship, and research funds ever, perhaps the extent of partial melting was not but they may have formed during greenschist from the University of Northern Colorado. IgPet 2013 software significant enough in the Highlands to allow such facies extension at ca. 945 Ma (Streepey (RockWare, Inc.) was used to construct the geochemical plots. dome or sub-dome structures to develop. et al., 2000). StereoWin software by Rick Allmendinger and Estereografica software by Ernesto Cristallini aided in stereographic analysis 6. Two models can explain the development and presentation. of the studied structures and other data sets ■■ CONCLUSIONS from the Adirondacks: a. Model 1 depicts the CCMZ as forming REFERENCES CITED There are a variety of conclusions regarding from top-to-the-SSE thrusting during Axen, G.J., Bartley, J.M., and Selverstone, J., 1995, Structural the origin of the Diana Complex and its deforma- Amazonia collision. The CCMZ was reacti- expression of a rolling hinge in the footwall of the Brenner tion history: vated at minimum during tectonic collapse Line normal fault, eastern Alps: Tectonics, v. 14, p. 1380– 1392, https://doi.org/10.1029/95TC02406. 1. The deformational history found in the as the Highlands exhumed, resulting in a Baird, G.B., 2001, Fabric analysis of the Diana Syenite within the southern CCMZ is significantly different symmetrical metamorphic core complex. Carthage-Colton Mylonite Zone, Northwest Adirondacks,

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