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Pseudostratigraphy and origin of the Early Payson ophiolite, central Arizona

Jesse C. Dann* Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri

ABSTRACT mua [Kontinen, 1987] and Portunique [Scott and tern and collection of diverse rock types reflect Bickle, 1991] are the other two ophiolites) with the simple magmatic pseudostratigraphy charac- The Payson ophiolite (ca. 1.73 Ga), exposed the well-developed sheeted complex and teristic of Phanerozoic ophiolites. within the Early Proterozoic of pseudostratigraphic structure characteristic of The original tectonic setting of an ophiolite central Arizona, is a shallow-dipping, pseu- Phanerozoic ophiolites (Moores, 1982). Their and mechanics of its emplacement into an oro- dostratigraphic sequence of , sheeted importance as hallmarks of sea-floor spreading genic belt are closely related. It is no coincidence dikes, and submarine volcanic rocks, partly and Early Proterozoic is high- that many ophiolites formed above disjointed by later intrusion and deformation. lighted by the controversy over the existence of zones (Pearce et al., 1984) because the setting of An older basement complex occurs as roof any older or ophiolites (e.g., Helmstaedt a convergent plate boundary predisposes them pendants in the gabbro and screens in the and Scott, 1992; Bickle et al., 1994; Sylvester et for being incorporated into dur- sheeted dike complex and was tilted and al., 1996). Unlike the Jormua and Portunique ing accretion or collision. All rocks of the Payson eroded prior to development of the ophiolite. ophiolites, the Payson ophiolite was not thrust ophiolite have similar patterns of light rare earth Gabbro-dike mingling and mutual intrusion over Archean basement, lacks a deformational element (LREE) and large ion lithophile element in the transition from gabbro to sheeted dikes fabric, and contains screens and roof pendants of (LILE) enrichment and relatively high field indicate that the dikes are rooted in the gab- older arc crust. As with every well-exposed ophi- strength element (HFSE) depletion and plot bro. Analysis of detailed measured sections of olite, the Payson ophiolite has unique or distin- within the arc or “suprasubduction zone” field on well-exposed sheeted dikes (600 m) reveals guishing features. Although some of these may all tectonic discrimination diagrams (Th-Hf-Ta, 90%Ð100% dikes with lateral variations in have significance for the conditions of sea-floor Ti-V, Cr-Y, etc., Dann, 1991a; 1992). The mafic the proportions of tonalitic or dacitic dikes spreading during the Early Proterozoic (e.g., dikes of the sheeted dike complex define a tholei-

and basement screens. The transition from crustal thickness, Moores, 1993), most probably itic fractionation trend of increasing FeOT,TiO2, sheeted dikes to submarine is abrupt reflect its formative tectonic setting within a P2O5, and V with decreasing MgO, typical of is- and marked by a stratigraphically continuous magmatic arc (Dann, 1991a). land arc tholeiites. Hydrous igneous mineralogy zone of intensely altered rocks. The sheeted Research on an ophiolite addresses three re- of the and clinopyroxene-controlled dike complex, pseudostratigraphic structure, lated problems: (1) recognition and definition, fractionation of the dikes (Dann, 1992) indicate a and synmagmatic hydrothermal alteration of (2) formative tectonic setting, and (3) mechanics hydrous parental expected for an arc set- the Payson ophiolite are diagnostic of a crustal of emplacement (and preservation) in an oro- ting. Nd isotopic analyses indicate the influence section formed by sea-floor spreading. genic belt (Moores, 1982). Ophiolites are de- of an older LREE-enriched component (Dann et The Payson ophiolite intrudes, and is in- fined by their pseudostratigraphy or horizontally al., 1993). The Payson ophiolite has the geo- truded by, arc granitoids and is overlain by layered structure (mantle , gabbro and chemical and isotopic features of the high Ce/Yb arc-derived volcanic and sedimentary rocks. sheeted dikes, and volcanic rocks) with transi- suite of arc (c.f., Hawksworth et al., A model of an intra-arc basin formed along an tions between lithologic layers, which indicate 1993). Although sheeted dikes may form during arc parallel strike-slip fault is proposed to ac- that the layers were forming at the same time the rifting of continental crust or rifting of count for (1) screens of older arc crust, (2) in- (unlike ). The horizontal pseudos- oceanic islands (e.g., Hawaii [Walker, 1987] and ferred arc-parallel extension, and (3) juxta- tratigraphy is a by-product of a vertical forma- the Canary Islands [Stillman, 1987]), the arc geo- position of the ophiolite with a distinct arc tive unit (gabbro, dike, and submarine lava flow) chemistry alone clearly indicates the Payson terrane prior to regional convergent deforma- structured by a steep temperature gradient and ophiolite formed during an extensional phase in tion and accretion of the arc to North America relatively abrupt rheological transitions. In the the evolution of an Early Proterozoic arc. during the Yavapai (ca. 1.70 Ga). Payson ophiolite, gabbro-dike mingling and mu- Emplacement of the Payson ophiolite may tual intrusion attest to “rooting” of sheeted dikes have selected a small piece from an originally INTRODUCTION in coeval gabbro in a rheologic transition zone more extensive basin (fore arc, intra-arc, or back from ductile flow of crystal-rich gabbroic arc) and juxtaposed it with unrelated rock se- The Payson ophiolite is one of only three Early magma to brittle fracture and dike intrusion. quences. This paper describes some of the criti- Proterozoic ophiolites reported worldwide (Jor- Secondary to the pseudostratigraphy, ophiolites cal field relationships that distinguish intrusive are characterized by synmagmatic hydrothermal versus tectonic contacts between the ophiolite alteration, extensional deformation, and a com- and older and younger arc rocks and that bear on *Present address: Department of Geological Sci- ences, University of Cape Town, Rondebosch 7700, ponent of tonalitic magmatism (plagiogranites). the recognition of possible terrane boundaries South Africa. E-mail: [email protected]. This paper describes how the complex map pat- and the timing of juxtaposition of distinct ter-

GSA Bulletin,; March 1997; v. 109; no. 3; p. 347Ð365; 14 figures.

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ranes relative to convergent deformation. The Range Provinces. In central Arizona, the oro- Most of the deformation along the block-bound- tectonostratigraphic history of the arc, how the genic belt consists of deformed submarine vol- ing shear zones reflects postassembly crustal ophiolite is situated within the orogenic belt, and canic rocks, or greenstone sequences, that host shortening and differential uplift (Bowring and its internal structure are consistent with a model massive sulfide deposits and are intruded by Karlstrom, 1990; Berg and Karlstrom, 1993). of an intra-arc basin, both formed and emplaced granitoid plutons (Fig. 1). These lithologic asso- Despite contrasting tectonostratigraphic histories along an arc-parallel strike-slip fault (Dann and ciations, interpreted by most workers to represent defining distinct terranes, no terrane boundaries Bowring, 1996). This model provides a useful magmatic arcs (e.g., Anderson, 1989), formed or sutures have been identified, and the mechan- working framework for relating unique or un- over a 40 m.y. period (1.75Ð1.71 Ga; Bowring et ics of accretion remain speculative. usual features of the ophiolite and its setting al., 1991), locally involving older crust. The pre- The Payson ophiolite is exposed within the within the orogenic belt. dominance of juvenile magmatic arc rocks in Mazatzal crustal block (Fig. 1), an area of low- central Arizona, the rapid rate of estimated to subgreenschist facies rocks, bound by a belt of TECTONIC SETTING crustal growth, and the juxtaposition of distinct 1.65 and 1.4 Ga postorogenic granite to the terranes suggest that the Early Proterozoic oro- southeast and the Moore Gulch shear zone to the Continental of the southwestern genic belt of the Southwest represents accretion northwest. The tectonostratigraphic history of United States formed by accretion of a broad belt of magmatic arc terranes, during three episodes the Mazatzal block is recorded by three distinct of Early Proterozoic crust to the southern margin of convergent tectonism at a convergent plate lithostratigraphic units (legend, Fig. 2; columnar of the Archean Wyoming Province (Fig. 1, inset). boundary (Karlstrom and Bowring, 1988). The section, Fig. 3): (1) the basement complex, A 600-km-wide transect of this Early Proterozoic orogenic belt is divided by north- to northeast- which is intruded and unconformably overlain orogenic belt is exposed in the Transition Zone trending shear zones into a collage of crustal by (2) the Payson ophiolite, which, along with a between the Colorado Plateau and Basin and blocks (Fig. 1; Karlstrom and Bowring, 1988). basin filling sequence of turbidites and volcani-

113° 0 10 20 30 40 50 112° 111° quartzite and rhyolite post-orogenic granite kilometers (1.66-1.62 Ga) (ca. 1.65 and 1.4 Ga ) quartzite (Mazatzal Group) u d Shylock ≤ N shear zone ( 1.70 Ga) x x subaerial rhyolite hypabyssal x syntectonic Bagdad x Jerome + + granite x qtz monz. x u d (1.70 Ga) x x x x Prescott 1.75 - 1.71 Ga magmatic arc rocks (Yavapai) xx x x submarine sediments x granodiorite x x x x x & volcaniclastic rocks x (tonalite, granite, diorite) x x x x x x submarine volcanic rocks x x 1.735 Ga tonalite x x x x x x x x x x x x Payson ophiolite x x x x x Ash Creek (1.73 Ga gabbro x x x x x block Moore Gulch & sheeted dikes) x x x x shear zone x 1.735 Ga x x x x Cherry batholith + Paleozoic sediments x x x x x x x x + + + + ++ + x + Fig. 2 + + ++x x x x x + x +x x + x x + x + ++ x x x + + + + + +++++ + x + + ++x+ x ? x x x + + + ++++ + x+ x x x x x + + + ++ + +++ + ++ + + + + ++++ + x x x x x + + + ++++++++ +++ x x + Payson + +++ + + + + + + ++ x x x + ++++ + x x ophiolite +++ x x + + + + x + + Mazatzal + + + + + x x 34° + + + x x + block + ° + + + u + + 34 + + + + + Big Bug + + + + + + x d + + + + + C T ++ + + Archean + block H + ++ + ++ + + + x u + + + ++ d + ++ + x + ++ + + ++ + + ++ + 1.1 Ga + 1.3-1.0 Ga Yavapai-Mazatzal Tertiary 1.6-1.8 Ga sedimentary 2.0-1.8 Ga and volcanic 2.5-1.8 Ga rocks ++ 113° ++ Archean 112° 111° North Phoenix

Figure 1. Geologic map of the Early Proterozoic rocks of central Arizona, showing the Mazatzal and Ash Creek crustal blocks and the north- east-trending Moore Gulch shear zone (modified from Karlstrom et al., 1987, and Anderson, 1989). Tonto Creek (T), Humboldt Mountain (H), and Cramm Mountain (C) are exposures of submarine volcanic rocks discussed in the text. Block boundaries have late west-side-up movement (u/d). Inset shows the location of the map (Fig. 1) in the Early Proterozoic Yavapai-Mazatzal orogenic belt of the southwest United States. (from Hoffman, 1989).

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0 1 2 3 4 5 km 1.73 Ga ophiolite

0 1 2 3 mi Tertiary volcanic rocks Snowstorm & Paleozoic sm sheeted dikes Mt. gabbro r D 3 ~ ~ granophyric dike swarm ~ r ~ ~ tonalite 75 v Mazatzal v d 1.73 Ga 82 v Group + diorite v 72 v x x x + hornblend 75 unconformity gabbro x x + granitic screens in + sheeted dikes Round Valley x a + r rv x + gabbro b ~ ~ 1.70 Ga granite ~ ~ & rhyolite Doll Baby layered gabbro ~ ~ ~ ~ D2 + granite AG x + + x 1.75 Ga Larson Spring fm. 1.71 Ga basement ~ felsic volcanic 1.75 Ga ~ + granodiorite ~ ~ ~ + + complex volcaniclastic coarse grained ~ ~ 77 granitoids ~ ~ ~ smsm + + rocks ~ ~ ~ ~ + + ~ 1.72 Ga thrusts Tertiary ~ ~ D D ~ turbidites 2 and folds 3 normal fault 85 c + sm sm + + + ~ ~ + + + + + Payson + + ~ ~ + sm + ++ + x' sm rv + + 42 + 56 52 + 60 69 + + + sm 49 rv 31 rv + 45 v + 37 65 65 sm v v + v x 75 40 76 + x + v x 54 + 58 SJ x x 68 74 + v x 48 23 60 d v 79 + x v x 53 70 RC x + d x EF x v X + 40 + + v 89 + + e

+ v

v XXv ' v v ? 5000 ft. + d rv ++

Figure 2. Geologic map and cross section of the Payson area, central Arizona (D3 thrusts and folds in Mazatzal Group from Puls, 1986, and Wrucke and Conway, 1987). American Gulch (AG), Rattlesnake Canyon (RC), east flank of the Mazatzal Mountains (EF), and St. Johns Creek (SJ) are separated areas of exposed sheeted dikes discussed in the text. East Verde River (a), Crackerjack Mine (b), Larson Spring (c), Center Creek (d), and Gisela (e) are areas of exposed Larson Spring formation of the basement complex intruded and overlain by the ophiolite.

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Payson map (Fig. 2) (ca. 1.75 Ga; Bowring, 1992, personal commun.) of the basement complex. This occurrence is iso- lated in the core of a faulted anticline within a NNW SSE stratigraphic Mazatzal Group shear zone (Wessels and Karlstrom, 1991) and time line thickness suggests that both the basement complex and 1700 1-5 ophiolite may extend under younger cover 1.70 Ga rhyolite throughout much of the Mazatzal block. East Verde turbidites The Payson ophiolite is overlain by an arc- River Fm. related -sedimentary sequence, charac- ? terized by a lack of stratigraphic continuity due to 1710 ash beds Alder Group both facies changes within the basin and tectonic 0.2 shortening. In the map area (Fig. 2), the basalts dacite are directly overlain by dacitic volcaniclastic 0 km ? ? breccia with lenses of jasper and a thick sequence 1720 of turbidites, both members of the East Verde 0.5 River Formation (Wrucke and Conway, 1987). + + + + Two air-fall ash beds within the turbidites have + + + + + U-Pb ages of 1717 ± 5 and 1717 ± 10 Ma (Dann + + + + sheeted + et al., 1989). Southwest of the map area (C and H 1730 dikes 1.75 Ga granite of in Fig. 1), turbidites are intruded by dikes feeding + + basement complex a thick sequence of andesitic flows and coarse ag- + 1.71 Ga glomerates and are in turn overlain by turbidites diorite granodiorite and pelitic sediments with ash beds (ca. 1.70 Ga; 1740 Bowring, 1992, personal commun.). Three small 2.0 bodies of granodiorite (1709 ± 6 Ma, Conway et Payson al., 1987) intrude the gabbro and sheeted dikes of ophiolite gabbro the Payson ophiolite (Fig. 2). These relationships Tonto Creek suggest that an andesitic arc volcano erupted be- 1750 10 km south and fore and during turbidite deposition (Anderson, granodiorite southeast of the map area of basement 1989) and on, or adjacent to, the marginal basin complex gabbro crust represented by the Payson ophiolite. In ad- dition, the Payson ophiolite was intruded by arc plutons (ca. 1.71 Ga), and arc volcanism and 3.5-4.0 1.70 Ga base not exposed basin sedimentation continued until shortly be- granite fore its exposure (ca. 1.70 Ga). Figure 3. Schematic columnar sections showing the relationship between the older basement The third unit of the Mazatzal block consists of complex, the suite of rocks assigned to the Payson ophiolite, and younger intrusive and sedi- caldera-related ash flow tuffs of the Red Rock mentary rocks of the Payson map (Fig. 2) and a related sequence of rocks in Tonto Creek (T in Rhyolite (ca. 1.70 Ga) and related hypabyssal Fig. 1). The time line shows U/Pb zircon ages with arrows to the unit analyzed, and the strati- granite (Conway et al., 1987) and fluvial to shal- graphic thickness is measured from the top of the submarine basalts, the contact between the low marine siliciclastic sediments of the Mazatzal ophiolite and overlying sediments of the East Verde River Formation. Group. These units unconformably overlie or in- trude the Payson ophiolite and East Verde River Formation and represent a fundamental change in the orogenic belt from submarine arc to more con- clastic sediments, is unconformably overlain by screens in the gabbro and sheeted dikes of the tinental styles of volcanism and sedimentation. (3) subaerial ash-flow tuffs of the Red Rock ophiolite, respectively. Prior to development of Of the three episodes of convergent deforma- Rhyolite and fluvial to shallow marine siliciclas- the ophiolite, the felsic volcaniclastic rocks and tion affecting central Arizona, only the last two tic sediments of the Mazatzal Group. These units granite of the basement complex were rotated are recorded in the Mazatzal block as structures are confined to, and define, the Mazatzal block. about a northeast-trending axis, and plutonic that predate and postdate a regional unconformity The ophiolite and basement complex of the rocks were exposed on the pre-ophiolite paleo- at the base of the Mazatzal Group. Deformation Mazatzal block are not compatible with the early surface that was later covered by submarine postdating the unconformity is due to the Maz-

tectonostratigraphic history of the adjacent Ash basalt. The basement complex and the pre-ophio- atzal orogeny (D3), and deformation truncated by Creek block to the northwest (Fig. 2) and define lite rotation are unique to the Payson ophiolite. the unconformity is due to the Yavapai orogeny

a distinct terrane within the orogenic belt. Unlike the Payson ophiolite, most sequences of (D2) (pre-ophiolite deformation in the Ash Creek The oldest rocks in central Arizona are grani- submarine volcanic rocks in the orogenic belt do block is pre-1.735 or D1). Although the ca. toids (1751 ± 3 Ma; Dann et al., 1989) of the not have exposed transitions to hypabyssal intru- 1.70 Ga unconformity occurs throughout the basement complex intruded by the Payson ophi- sive equivalents. The only other exception is Mazatzal block, fold and thrust deformation pre-

olite (1729 ± 6 Ma; Dann et al., 1989) exposed in south of the map area on Tonto Creek (T in dating the unconformity (D2) is most clearly ex- the central part of the Mazatzal block. The base- Fig. 1) where pillow basalts overlie a thin sheeted posed in the map area (Fig. 2). The unconformity ment complex occurs as roof pendants and dike complex and mafic dike swarm in granitoids truncates both limbs of a shallowly plunging,

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northwest-verging, upright D2 syncline in the tur- the older sequence of the Ash Creek block. Al- ophiolite. The Round Valley gabbro in the north- bidites and cuts down section to the sheeted dikes ternatively, the Mazatzal and Ash Creek blocks eastern part of the map area is completely devoid

(Fig. 2). The D2 syncline is truncated by an east- represent distinct terranes juxtaposed prior to of mafic dikes and is generally coarse grained dipping D2 thrust fault, and both structures are in- convergent deformation (D2ÐD3), possibly along with the least alteration of primary igneous min- truded by hypabyssal feeders and intrusions re- cryptic strike-slip faults. eralogy (1%Ð5%). The northeast-southwest lated to the subaerial ash flow tuffs of the Red cross section (inset, Fig. 2) shows that the Round Rock Rhyolite (ca. 1.70 Ga). Erosion removed all THE PAYSON OPHIOLITE Valley gabbro solidified about 1.5Ð2 km below of the rhyolite from this area prior to deposition the base of the sheeted dike complex. The unit of of the Mazatzal Group, except for a few down- The basic structure of the ophiolite is reflected hornblende gabbro (Fig. 3) mantles the Round faulted blocks and small channels filled with in the distribution and orientation of the dikes. Valley gabbro, and together these gabbros form blocks of rhyolitic breccia. The unconformity is Sheeted dikes and mafic dike swarms in gabbro a well-vegetated physiographic plateau cut by marked by extensive hematitic alteration of the form a central domain that lies between subma- drainages with abundant water-polished out- underlying rocks, and concretionary, hematite- rine volcanic rocks to the west and gabbro com- crops. Toward the northwest-trending transition rich nodules, sand-filled mud cracks, and asym- pletely devoid of dikes to the east (Fig. 2). The to the sheeted dike complex, the hornblende gab- metric ripple marks in overlying ferruginous sed- sheeted dikes strike northwest with an average bro is increasingly finer grained, fractured, and iments attest to subaerial conditions of an dip of about 75¡ to the northeast. The northeast- altered, and mutually intrusive relationships in- intermittent fluvial system. Similar ages from to-southwest sequence of gabbro, sheeted dikes, dicate that the sheeted dikes are rooted in the submarine ash beds below the unconformity and volcanic rocks, combined with the 75¡ north- hornblende gabbro. The Snowstorm Mountain (Bowring, 1992, personal commun.) and subaer- east dip of the dikes, suggests that the sheeted gabbro intrudes the sheeted dike complex, cut- ial rhyolite above the unconformity (Conway et dike complex and the ophiolitic pseudostratigra- ting across the trend of the dikes and dividing it

al., 1987) indicate that D2 deformation occurred phy dips about 15¡ to the southwest (cross sec- into three domains (Fig. 2). The diversity of frac- ca. 1700 Ma during the Yavapai orogeny. Based tion, Fig. 2). The Payson granite (ca. 1.70 Ga) in- tionated rock types and high An content of its on the fundamental change from submarine to trudes parallel to the ophiolite pseudostratigraphy plagioclase are distinguishing features of this continental styles of volcanism and sedimenta- as a sheet dipping 15¡Ð25¡ southwest below the gabbro. Rare mafic dikes intrude the Snowstorm tion and the regional extent of the 1.70 Ga defor- Round Valley gabbro (Conway et al., 1987) and Mountain gabbro parallel to the trend of the host mation, the deformation and subaerial exposure intrudes its gabbroic roof as rhyolitic and gra- sheeted dike complex, suggesting that it intruded (ca. 1.70 Ga) of the Payson ophiolite is inter- nophyric dikes. Due to the shallow dip of both off the axis of dike intrusion as a distinct phase preted to have coincided with the accretion of the the ophiolitic pseudostratigraphy and the sheet of of the ophiolitic magmatism. A sill of entire arc to the North American continent, dur- granite, hornblende gabbro-norite of Round Val- and gabbro (Alder Creek, Fig. 3) intrudes the ing the Yavapai orogeny. ley (Fig. 2) is the deepest exposed level of the submarine volcanic rocks (too small to show in Fold and thrust deformation of the Mazatzal ophiolite. Tertiary normal faults have created the Fig. 2). None of the exposed gabbroic bodies are

orogeny (D3) is essentially coaxial with the ear- sediment-filled valley (Fig. 2) and the compli- the cumulates complementary to the fractiona- lier deformation below the unconformity, over- cated map pattern by juxtaposing different levels tion trend in the sheeted dikes (Dann, 1992) and printing and reactivating earlier structures (Dann, of the ophiolitic pseudostratigraphy. The follow- are high level gabbros with respect to the com- 1991b), folding the unconformity, and producing ing description of the ophiolite proceeds from plete ophiolite column (Moores, 1982). 35%Ð40% shortening in the Mazatzal Group gabbro to volcanic rocks with particular empha- Magmatic Layering. The gabbroic rocks of (Puls, 1986). Most of the Payson ophiolite ex- sis on the transitions that establish the temporal the Payson ophiolite range from coarse-grained, posed in the map area lies within the foot wall of connections between the different lithologic lay- well-layered and/or foliated gabbro to finer

the Mazatzal (D3) thrust system and has escaped ers of the ophiolite. grained, more -rich, isotropic phases structural imbrication and duplication. (Fig. 4). In the Doll Baby layered gabbro, sharp The block-bounding shear zones record late Gabbro contacts at the base of the layers crustal shortening and differential uplift of the (≤5 m) and consistent, northwesterly modal grad- blocks, not terrane accretion. Northwest-side up Gabbro solidified, not only as a pseudostrati- ing to anorthosite layers suggests that this ver- movement along the Moore Gulch fault locally graphic layer of plutons below the sheeted dike tically dipping cumulate sequence faces west-

truncates D3 fold axes in rhyolite and quartzite complex of the Payson ophiolite, but also as dis- northwest. Some thin layers of pyroxenite were overlying the ca. 1.70 Ga unconformity (Dann, tinct bodies intruding the sheeted dike complex folded (Fig. 5A) prior to solidification of this cu- unpub. mapping) and juxtaposes ca. 1.735 Ga and volcanic section. Major gabbroic units are mulate sequence. In the Round Valley gabbro, a arc plutonic rocks of the Ash Creek block with located on the map with some representative ori- foliation defined by the alignment of plagioclase ca. 1.70 Ga hypabyssal granite of the Mazatzal entations of layering and magmatic foliation and wispy modal layering trends northeast with a

block. As a result of this post-D3 movement, the (Fig. 2) and shown schematically on a columnar variable dip, and distinct planar modal lamina- contact between the 1.73 Ga ophiolitic rocks and section along with a table summarizing their tions dip moderately to the southeast (Fig. 2). Lo- the 1.735 Ga arc batholith of the Ash Creek specific rock types, structures, textures, igneous cally, small folds of the wispy layering formed block is tectonic and/or remains covered. In ad- mineralogy, and secondary alteration (Fig. 4). prior to solidification of the gabbro. The horn- dition, deformed ca. 1.71 Ga sediments traced The Doll Baby layered gabbro is a vertically dip- blende gabbro has a moderate to strong foliation across (overlapping) several shear zones (Ander- ping sequence of distinctly interlayered pale and lineation (northeast-trending) defined by the

son, 1989) suggest that D2 crustal shortening did anorthosite and black pyroxenite (Fig. 5A), alignment of plagioclase and, in some places, not involve large displacements along these which is intruded by a mafic dike swarm and highlighted by wispy modal layering. The folia- shear zones, and therefore, the younger subma- granophyric tonalite. Distinct vertical layering tion and lineation in the gabbro intruded by the rine volcanic sequence of the Mazatzal block and high degree of fractionation distinguish this mafic dike swarm are generally orthogonal to the may have been deposited on the eastern flank of gabbro from all other exposed gabbros of the dikes, but in the gabbroic screens in the sheeted

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submarine volcanic rock types structure texture igneous mineralogy alteration section (0.4 km) Alder Creek gabbro diabase, hbd gabbro isotropic poikolitic hbd plag+hbd+mag+qtz only texture preserved qtz-diorite screens qtz diorite isotropic granular plag+hbd+qtz+mgt sheeted dike hbd gabbro weakly layered & foliated; ophitic hbd plag+hbd+cpx+mgt±zc+sph complex anorthositic gabbro isotropic, layered; (1-2 cm) plag (An )+hbd 1-5% (1-1.5 km) Snowstorm Mountain 86 gabbro qtz gabbro isotropic, intrusion breccia; granular plag (An57)+hbd+qtz pegmatite crudely layered; plag+hbd+qtz

± small diorite plutons (qtz) diorite isotropic, porphyritic or plag (An<50)+hbd qtz 80-90% intrusion breccia granular +mgt+zc dike swarm ± hbd gabbro strongly foliated/lineated, poikolitic hbd plag (An50-66)+hbd cpx 10-80% hornblende gabbro wispy modal layering≤ 1 cm ±bio+qtz+mgt+ap±zc±sph wmc+act+chl 0.8-1.2 km locally folded +ept+prn

SE dipping gabbro without hbd gabbro, planar modal layering; ophitic hbd plag (An62-70)+hbd dikes Round Valley gabbro hbd gabbro-norite foliated, wispey ≤ 3 cm +cpx+mgt+opx+qtz 1-5% mgt-rich layering locally folded +ap±stlp chl+act

0.5-0.6 km vertically dipping only texture Doll Baby anorthosite (ant), interlayered pxt/ant, cumulate plag (alt: wmc) preserved layered gabbro pyroxenite (pxt) NW grading from pxt to ant, intersertal hbd px (alt: act+chl+serp+) to (ol websterite) locally folded layers cpx+ol+opx+hbd 60% anorthosite pyroxenite

Figure 4. Gabbroic rocks of the Payson ophiolite shown on a schematic columnar section with corresponding table summarizing rock types, magmatic structures, textures, and primary and secondary mineralogy. Abbreviations: act—actinolite, ap—apatite, bio—biotite, chl—chlorite, cpx—clinopyroxene, ept—, hbd—hornblende, mgt—magnetite, ol—olivine, opx—orthopyroxene, plag—plagioclase, prn—prehnite, qtz—quartz, serp—serpentine, sph—sphene, stl—stilpnomelane, wmc—fine-grained white mica, zc—zircon.

dike complex, they vary from orthogonal to tinctly smaller than their counterparts outside the The increasing degree of preservation of the ig- nearly parallel to the dikes. Defined by higher hornblende. Alteration to chlorite and green am- neous minerals in gabbros below the sheeted dike proportions of plagioclase or mafic minerals, the phibole affects less than 5% of the gabbro. Where complex indicates (1) that equilibration was in- wispy layering is thin (≈1 cm), discontinuous dikes are abundant in the Doll Baby layered gab- complete both to hydrothermal metamorphism (over 2Ð3 m), and locally deformed into asym- bro, alteration is 100%, and where dikes are rare, during sea-floor spreading and to later low-green- metric folds and rootless fold noses (Fig. 5B). olivine, clinopyroxene, orthopyroxene, and brown schist grade regional metamorphism, and (2) that Thin sections of the most well-foliated gabbro re- hornblende are partly preserved in an olivine web- the hydrothermal metamorphism during sub- veal elongate plagioclase crystals stacked in par- sterite. The map unit hornblende gabbro is vari- solidus cooling of the gabbro, dependent on frac-

allel. Some crystals have undulose extinction and ably altered. Calcic plagioclase (An50Ð66, rarely ture-controlled permeability and proximity to the bent lamellae or are bent and even broken apart. zoned) is partly altered in every sample to fine- sheeted dike complex, produced most of the alter- Grain size reduction occurs locally along the crys- grained white mica. With increasing alteration, ation. The Snowstorm Mountain gabbro escaped tal-crystal contacts. Supersolidus minerals like the pale brown, poikilitic hornblende becomes alteration because it intruded the sheeted dike quartz and magnetite are found filling spaces be- green or blue green, and then recrystallizes into a complex, late or off the axis of dike intrusion and tween broken plagioclase crystals (Fig. 5C), indi- patchwork of separate domains. Some green hydrothermal activity. cating that they were aligned and locally broken, hornblende contains clinopyroxene cores, and prior to solidification of the gabbro. The folding optically continuous, isolated remnants of clino- Transition from Gabbro to Sheeted Dikes of the wispy modal layering and the orientation of within hornblende indicate that the the foliation and lineation of the gabbro indicate hornblende grew partly at the expense of the The transition from gabbro with a few dikes to the flow of crystal-rich magma parallel to the di- clinopyroxene. Biotite occurs as a primary mag- the sheeted dike complex (≥90% dikes) trends rection of extension beneath the sheeted dike matic mineral, as well as small secondary mats northwest parallel to the dikes and is best ex- complex (i.e., magmatic flow foliation). that overgrow hornblende, and is commonly al- posed in the Rattlesnake Canyon area (RC in Igneous Mineralogy and Hydrothermal Al- tered to chlorite. Both alteration of the gabbro and Fig. 2). Besides the change in the spacing of teration. Preservation of igneous minerals is the density of mafic dikes and fractures increase dikes (from ≤50% to ≥90% dikes within 100 m), nearly ubiquitous in the gabbroic rocks that have toward the sheeted dike complex. Screens of gab- the transition contains mafic xenoliths, small not been intruded by dikes—the Snowstorm bro are the most altered and fractured, but some dioritic intrusions, gabbro-dike mingling fea- Mountain and Round Valley gabbros (Fig. 4). calcic plagioclase is commonly preserved. Diorite tures, and extensional shear bands. The north- Most samples of these gabbros contain pale intruding the gabbro and intruded by dikes at the west-trending dike swarm is dominated by mafic brown poikilitic hornblende (≤3 cm) that includes base of the sheeted dike complex is 100% altered dikes that vary in density from sparse to sheeted unaltered grains of plagioclase, clinopyroxene, (plagioclase is replaced 100% by fine-grained across a 20Ð30 m interval. The mafic dikes vary and magnetite (±orthopyroxene) that are dis- phyllosilicates and euhedral hornblende is green). texturally from aphyric to plagioclase- and/or

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A B

C D

Figure 5. (A) Interlayered pyroxenite (black) and anorthositic gabbro (white) cut by a mafic dike (arrows mark the chilled margin) in the Doll Baby layered gabbro. Note the folded layer (≈15 cm thick) of pyroxenite. (B) Folded modal layering in foliated hornblende gabbro (pen for scale; arrow points to fold nose). (C) Photomicrograph (3 mm wide) of foliated gabbro with spaces between broken plagioclase (P) filled by late crystallizing magnetite (M). (D) Margin (marked by arrow) of dioritic dike (1.5 m wide) chilled against texturally identical isotropic hornblende diorite (scale marks 3 cm).

pyroxene-phyric and from aphanitic to diabasic grained, foliated gabbro over a complex intru- porphyritic diabase are locally abundant in the and are pervasively altered to greenschist. Sev- sive zone locally marked by intrusion . gabbro near the sheeted dike complex. The eral sheeted intervals include very wide (10 m) The diorite locally contains abundant xenoliths xenoliths are texturally identical to nearby dikes tonalitic dikes split by later tonalitic and mafic of mafic dikes and is in turn cut by later dikes intruding the gabbro. Trains of xenoliths occur dikes. The tonalitic and mafic dikes in the dike parallel to the sheeted dikes. Interestingly, this along the foliation (Fig. 6B), and some xenoliths swarm are texturally and geochemically similar diorite contains a relatively narrow dike (1.5 m) have convex inward lobes and cusps and others to those in the sheeted dike complex. of diorite that is texturally indistinguishable are flattened, elongate, and fragmented along the The northwest-trending boundary zone be- from the host rock except for thin chilled mar- foliation. As the basement complex intruded by tween gabbro and sheeted dikes contains at least gins parallel to the dike swarm (Fig. 5D). The the gabbro does not contain basalt, the xenoliths six distinct plutons of isotropic diorite (lateral coarse texture of this dike indicates slow cooling are probably fragments of mafic dikes that were extent ≤1.5 km). The dioritic plutons are distin- within hot subsolidus diorite undergoing exten- engulfed, pulled apart, and plastically deformed guished from the host gabbro by (1) finer grain sion. Collectively, these features suggest that the by continued flow of crystal-rich gabbroic size, (2) an isotropic fabric, and (3) in some, a diorite intruded the base of the sheeted dike magma. granular or “salt and pepper” texture due to eu- complex during its development. The 1729 ± 6 Gabbro in the transition to, and as screens hedral hornblende, and in others, a porphyritic age of this pluton, therefore, is a reasonable esti- within, the sheeted dike complex is intruded first texture, resembling the cores of thick dikes. One mate of the age of the ophiolite as a whole (Dann by fine-grained diabase that lacks the distinct of these plutons at the base of the sheeted dike et al., 1989). chilled margins characteristic of the later dikes. complex (d in Fig. 2) intrudes the coarser Dark green-gray xenoliths of fine-grained The diabase is texturally similar to the dikes or

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intermediate between the dikes and fine-grained northwest A. Figure 6. Gabbro-dike mingling diorite and distinct from the much coarser and synmagmatic strain with in- grained gabbro. This diabase takes the form of creasing depth below the sheeted irregularly formed, unchilled intrusions (proto- gabbro-diabase dike complex. (A) Schematic block dikes, Fig. 7B) parallel to the later dikes and mingling diagram showing mingling be- complex mixtures of gabbro and diabase that tween gabbro (pale shades) and di- have a fabric, contacts, or layering, roughly or- mafic abase (gray) cut by mafic dikes thogonal to the dikes and parallel to the flow lin- dike (dark shades) at the base of the eation or foliation in the gabbro (Fig. 7A). One sheeted dike complex, Rattlesnake margin of a proto-dike (387 cm) is chilled and Canyon area (scale: edge of block is simple, and the other has a zone of mingling plunge of about 2 m). The lineation (arrow) is flow lineation (80 cm wide) between the fine-grained dike ma- defined by the elongation of the terial and the coarse-grained gabbroic screen 0.5 m shapes and alignment of plagio- (see z in Fig. 8). The gabbro occurs both as xeno- clase in the gabbro. (B) Planar liths within the dike and as the matrix around B. trains of mafic xenoliths (dark fragments of the intruding dike. The later well- shapes marked by arrows), parallel chilled sheeted dikes rarely contain gabbroic or to the flow foliation (parallel to ar- other xenoliths. Where the host gabbro is well basaltic rows) in hornblende gabbro (trace foliated, it locally contains elongate inclusions enclaves in gabbro of outcrop photograph), are inter- of dike material along the contact with diabase preted to be dikes engulfed by flow- that contains elongate inclusions of gabbro. The ing crystal-rich gabbro. (C) Block contact as well as the fabric defined by the elon- C. diagrams summarizing the orien- gation and/or flattening of the inclusions are tation and kinematics of exten- nearly orthogonal to the sheeted dikes (Fig. 6A). sional, brittle-ductile shear bands The mutually intrusive relationship between the extensional, brittle-ductile, in flow-foliated gabbro intruded by diabase and gabbro, the lack of chilled margins, shear bands some mafic dikes. and the common flow foliation/lineation indicate that the diabase represents basaltic magma that intruded, and mixed with, cooler, partly molten and mobile, crystal-rich gabbro. of dikes is low, brittle-ductile shear bands are compilation of more than 600 m of measured Similar features occur in other ophiolites. For recognized by the foliation curving into thin section (selected examples, Fig. 8), which permit example, rooting of the sheeted dikes in the foli- (<1 cm) cataclastic seams (Fig. 6C). The shear an estimate of the proportions of dacitic and ated gabbro of the ophiolite involves un- bands strike parallel to, and dip less steeply than, mafic dikes and screens of gabbro and granitic chilled intrusions, mutual intrusion, and dike the few dikes found in same area. The direction basement and other features that distinguish one material stretched into the foliation during flow of movement on conjugate shears is consistently area of sheeted dikes from another (Fig. 10). Al- (Nicholas and Boudier, 1991). The transition in extensional with reference to originally vertical though no complete section, continuous from the Karmoy ophiolite is characterized by multi- dikes and is compatible with the shear bands, bottom to top of the sheeted dike complex, is ex- ple intrusive phases of gabbro, dike xenoliths, magmatic fabric in the gabbro, and the dikes be- posed, different areas may expose up to 0.25 km and microgabbroic dikes that were emplaced be- ing products of the same extensional stress of section from different depths within the fore the host gabbro was fully solidified (Peder- regime. Extension of hot subsolidus gabbro dur- sheeted dike complex. The base of the sheeted son, 1986), and early unchilled irregular intru- ing rifting is the simplest explanation for the dike complex and transition to the underlying sions into partly molten gabbro occur in the brittle-ductile shear bands. gabbro is exposed in the Rattlesnake Canyon sheeted dike complex of the Leka ophiolite (RC) area (Fig. 2). The top of the sheeted dike (Furnes et al., 1988). Complex contact relations Sheeted Dike Complex complex and transition to volcanic rocks is only between texturally variable gabbro, diorite, dia- exposed on the other side of the Tertiary sedi- base, and mafic dikes are well documented for The sheeted dike complex is exposed in four ment-filled valley along the east flank (EF) of the gabbro-sheeted dike transition in the Solund- major drainages as continuous stream-polished the Mazatzal Mountains (Fig. 2). American Stavfjord ophiolite (Skjerlie and Furnes, 1996). outcrop (Fig. 7C), cliffs of parallel slabs of dikes Gulch (AG) provides the best display of the in- The proto-dikes and mingling features in the (Fig. 7D), and hillsides of dikes standing in re- trusive sequence of basement screens and dacitic Payson ophiolite indicate that the sheeted dikes lief. The sheeted dike complex consists of sub- and mafic dikes and exposes an intermediate are rooted in the hornblende gabbro and provide parallel intrusions of 80%Ð100% dikes with level within the sheeted dike complex. The an elegant temporal and genetic link between well-defined chilled margins and with crosscut- sheeted dike complex can be understood as a these two distinct forms, dikes and gabbro, as- ting contacts limited to low angles (Fig. 9). As a whole by assembling the sheeted dike sections sumed by the same mafic magma. Mingling of result of the high density of dikes, most dikes are and distinguishing features with a vertical gradi- mafic and crystal-rich granitoid magma occurs chilled against other dikes, and dike splitting can ent within the sheeted dike complex from those in many plutonic belts (e.g., Foster and Hynd- displace segments of a single dike over tens of with lateral variations. man, 1990; Dorais et al., 1990), but gabbro-dike meters. Only by matching chilled margins and The composition of the sheeted dike complex mingling in the transition from gabbro to sheeted accounting for all the septa can the width of is distinctly bimodal with basaltic predominant dikes is a unique feature of ophiolites. dikes and intrusive sequence be determined. The over dacitic dikes (60:1 to 6:1, Fig. 10). The In the hornblende gabbro where the spacing quality of exposure inspired and facilitated the chilled margins of some dacitic dikes, as well as

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A B

C D

Figure 7. (A) Screen of gabbro-dike mingling (center, 50Ð75 cm across) in sheeted dikes (arrows mark chilled margins) of the Rattlesnake Canyon area, which consists of mixed gabbro (palest granular phase) and green finer grained diabase (darker phases). (B) Proto-dikes (≈20 cm wide) intruding en echelon fractures and reshaped by isotropic gabbro. (C) Sheeted dikes exposed in canyon walls of American Gulch (AG in Fig. 2). (D) Sheeted dikes with granitic screens (pale), water polished in Rock Creek (EF in Fig. 2).

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x dacite 1 2 m

B granite

gabbro y

C z

Figure 8. Selected measured sections from top (A) to base (C) of the sheeted dike complex. (A) 100% sheeted dikes (from EF in Fig. 2) with screen of dacitic dike (x). (B) Section within the sheeted dike complex (from AG in Fig. 2) with a high proportion of granitic screens intruded by gabbro. Melt generated along the gabbro-granite contact (y) crosscuts a mafic dike as an intrusion breccia. (C) Section of sheeted dikes (from RC in Fig. 2) shows gabbroic screens intruded by proto-dike with dike-gabbro mingling along one margin (z). Note the thick diabase intruding gabbro along an unchilled contact.

the thin dikes, are dark brown and black and ap- (≈1%). However, large areas of the top of the eral assemblages, and pyroxene is sporadically pear almost glassy, but generally the wider daci- sheeted dike complex, not represented by the preserved. Sulfides occur as disseminations and tic dikes are pale pink and/or green and stand out sections, are also devoid of basement screens as veinlets, and quartz veins fill fractures espe- in contrast to the dark gray to greenish-gray and dacitic dikes. The distribution of basement cially along dike margins. The interiors of some mafic dikes. Basement screens of pink granite screens and felsic dikes, therefore, reflects lat- dikes with identifiable chilled margins are com- (Fig. 9) mark chilled margins and provide a ref- eral variations rather than a vertical gradient in pletely altered to epidote and quartz. These epi- erence for determining the degree of extension the sheeted dike complex. dosites are locally intruded by gray, less altered and sequence of intrusion of dikes. The sheeted Hydrothermal alteration has affected the en- dikes. Some dikes with a reddish-brown, hema- dikes of the Rattlesnake Canyon area are devoid tire sheeted dike complex. Typically, original ig- titic alteration are split by succeeding dikes with of basement screens, and tonalitic dikes are rare neous textures are preserved by greenschist min- the more usual greenschist alteration. This oxi-

315 77

strike & dip of dikes (ave.)

0 1 2 3 m

sheeted dikes of American Gulch quartz-filled fault chilled granitic screens dacitic dikes diabasic dikes basaltic dikes margin

Figure 9. Outcrop map of sheeted dikes in American Gulch (traced from photographs taken from opposite canyon wall).

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granite east flank some outcrop areas have only aphyric dikes and others have many porphyritic dikes, both early dacitic and late. As a result, the concentration of phe- dikes nocrysts varies laterally within the sheeted dike complex and complements a similar distribution American Gulch in the volcanic section. Amygdules and other open spaces filled with calcite are common in mafic dikes just below the transition to volcanic rocks along the east flank. Amygdules filled with calcite or quartz and chlo- Rattlesnake Canyon rite form layers in the interior of dikes parallel to the margins. Rarely, amygdules are elongate due to flow. Gashes orthogonal to, and terminating against, the margins of some dikes formed dur- Oxbow Hill dike swarm ing solidification of the dikes and are filled with calcite. Amygdules and gashes are restricted to dikes along the east flank and indicate the dikes gabbro mafic & tonalitic dikes solidified under less pressure, as expected for the top of the sheeted dike complex. Screens occur throughout the sheeted dike 0 20406080100 0 100 300 complex except near the transition to volcanic average width of percentage of measured sections mafic dikes (cm) basalt micro-d diabase gabbro rocks. At the base of the sheeted dike complex, texture (qualitative) the screens are coarse grained, flow-foliated Figure 10. Proportion of rock types for each area (determined from measured sections) gabbro with proto-dikes and gabbro-dike mingl- arranged by depth in the complex with corresponding average width and texture (qualitative ing features that temporally link solidification of grain size assignment) of mafic dikes. the gabbro and intrusion of the dikes (Figs. 6, 7, 8C). In American Gulch (Fig. 9), granitic base- ment is fragmented into elongate slivers by low- angle cross cutting and undulation of the sheeted dizing alteration occurs only at the top of the and sparse phenocrysts, they could be mistaken dikes. Granite is intruded by isotropic quartz sheeted dike complex (EF). Based on these cross for gabbroic screens. In addition, the tonalitic diorite or gabbro, and the contact is intruded by cutting relationships, hydrothermal alteration is dikes are coarser grained than compositionally mafic dikes (Fig. 8B). Melt generated locally temporally linked with intrusion of the sheeted equivalent dacitic dikes in the other areas. In along this contact forms small intrusion breccias dikes. American Gulch (Fig. 9), early porphyritic dikes that intrude and fragment the mafic dikes (y in Besides proximity to the transitions to vol- have irregular contacts with the granitic screens; Fig. 8B). Stoping and assimilation of the granitic canic and gabbroic rocks, the most striking dif- thick, distinctly parallel, aphyric diabasic dikes basement by gabbro may have produced the in- ference between the top and bottom of the (average = 255 cm, n = 9) account for most of termediate magma compositions. Dikes of por- sheeted dike complex is the threefold increase in the extension; and a late set of relatively thin phyritic tonalite intrude the granite and gabbro average dike width (Fig. 10). The width of any basaltic dikes (average = 80, n = 6) include the and, where they occur as narrow screens be- single dike is not diagnostic of its depth, but de- most primitive and evolved compositions. Gen- tween the mafic dikes, they appear similar to the spite the large standard deviations about the aver- erally, the texturally coarsest dikes in any one granite, except for the porphyritic texture and age, relatively small isolated outcrops of sheeted area are the widest. Even the widest basaltic and lack of alkali . These early tonalitic dikes dikes from EF (top) and RC (base) would almost dacitic dikes (4 m and 6 m, respectively) along have the same northwest-trending orientation always be distinguishable solely on the basis of the EF (top), however, are very fine grained, in- and composition as the succeeding dacitic dikes average dike width (running averages of dike dicating more rapid cooling of the dikes at the and are an intrinsic part of the sheeted dike com- width for intervals averaging 8 to 9 m (approxi- top of the sheeted dike complex than at the base. plex. Screens along the EF (top) are quartz dior- mate outcrop size) across the measured sections Generally, the concentration and size of phe- ite and granite and felsic volcanic and volcani- for the two areas do not overlap). nocrysts in the chilled margins is distinctly less clastic rocks of the basement complex. The Grain size of aphyric dikes or the groundmass than in the center of mafic dikes. In fact, the MgO quartz diorite probably intrudes the basement of porphyritic dikes reflects the rate of cooling, content doubles from the margin to the center of complex and both are intruded by dacitic dikes which depends on both dike width and depth some porphyritic dikes due to the concentration and then the onslaught of mafic dikes (Fig. 8A within the sheeted dike complex (Fig. 10). Tex- of pyroxene phenocrysts (Dann, 1992). The phe- shows the mafic sheeted dikes chilled against a turally, the sheeted dikes are aphyric to plagio- nocrysts in some dikes show a size grading in screen of a dacitic dike that, out of the section, is clase- and/or pyroxene-phyric basalt, diabase, layers parallel to the margins. The distribution of chilled against quartz diorite). In summary, the and gabbro, and the felsic dikes are aphyric to phenocrysts reflects the flow and fractionation of intrusive sequence for the sheeted dike complex plagioclase- and hornblende-phyric dacite, gra- magma in the dikes (flow differentiation). Gener- is (1) gabbro or quartz diorite, (2) proto-dikes nophyric tonalite, and tonalite. The dikes in the ally, the size of the phenocrysts increases with (base only), (3) dikes of porphyritic tonalite, and RC (base) area are diabase and gabbro, and the grain size and depth in the sheeted dike complex. in American Gulch, (4) porphyritic mafic dikes, interiors of some dikes are so course grained Although, porphyritic dikes are commonly found (5) dacitic dikes, (6) diabasic dikes, (7) dacitic that, without the fine-grained chilled margins as screens intruded by later more aphyric dikes, dikes, and (8) basaltic dikes.

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Sheeted Dike Complex as Whole. The top and the base of the sheeted dike complex are ex- posed on opposite sides of a Tertiary valley 0 (Fig. 2). The thickness of the sheeted dike com- plex is probably between 1.5 and 2 km, however, an exact determination is hampered by unknown offsets on Tertiary normal faults (outcrops of Pa- 100 m leozoic sediments occur only on the east side of few late dikes penetrate part way the valley) and uncertainties due to folding along into massive altered rock the east flank. The discontinuous exposure of the sheeted dike complex across the map area 86 cm 67 120 40 80 (Fig. 2) might suggest that the sheeted dike com- 200 m plex was not a continuous pseudostratigraphic layer. However, roof pendants of sheeted dikes in a younger ca. 1.71 Ga granodiorite in St. Johns Creek (SJ in Fig. 2) are situated between the three major areas of exposed sheeted dikes. early dikes with locally preserved chilled margins 300 m The granodiorite has marginal intrusive contacts are variably silicified and altered with gabbro and a dike swarm (≤50% dikes), and >130 cm 61 106 150 the roof pendants must be remnants of the sheeted dike complex overlying the gabbro and eroded from the surrounding area. The roof pen- 400 m dants of sheeted dikes provide evidence for the original pseudostratigraphic continuity of the sheeted dike complex for a minimum across strike width of 22 km. early porphyritic dikes and late aphyric, amygdaloidal 500 m Overall, the sheeted dike complex is charac- dikes chilled against granitoid terized by a vertical gradient in dike width and grain size and lateral variations in the propor- Figure 11. Diagram of outcrops along a traverse up Center Creek (d in Fig. 2) toward a large tions of tonalitic or dacitic dikes, basement waterfall outcrop of the altered transition from sheeted dikes to submarine basalt. Sheeted screens, and concentration of phenocrysts. dikes with central trains of amygdules and calcite-filled gashes are chilled against granitic Proto-dikes and gabbro-dike mingling features screens (wide rule) and give way to 100% sheeted dikes (narrow rule) with strongly altered in gabbroic screens only occur at the base of the early dikes. The stratigraphically continuous unit of resistant, massive, altered rock (white) sheeted dike complex, and amygdules and other marks the boundary between sheeted dikes and volcanic rocks (black). open-space fillings and hematitic alteration only occur at the top. The increase in average dike width with depth suggests that the individual dikes either taper or branch upwards in the small patches of gossan creates orange wards into the massive, green, siliceous rock sheeted dike complex. staining on some cliff exposures. The massive from which all primary characteristics of mafic siliceous unit is steeply dipping to overturned on dikes or flows have been obliterated. The sili- Transition to Submarine Basalts and the northwest-facing limb of a syncline cored by ceous zone is predominantly massive with a few Hydrothermal Alteration the turbidites and in the hanging wall of the east discontinuous lenses of a well-bedded cherty flank thrust (see Fig. 12). The unit is internally sediment. Unusual breccia textures consisting of The transition from sheeted dikes to volcanic disjointed by conjugate fractures and faults with dark, wispy shapes of chlorite-rich material in a rocks is particularly interesting because it de- small displacements. A nonpenetrative, crude light siliceous matrix are locally prominent. A fines the pseudostratigraphy characteristic of cleavage evident only on weathered outcrops is true breccia of volcanic material in a siliceous ophiolites and marks the site of eruption on the better developed in this unit than in the mafic matrix occurs at the top of the unit, and the pres- sea floor (Kidd, 1977). In the Payson ophiolite rocks that bound it or in the turbidites. As a result ence of some bedded cherty or tuffaceous sedi- along the EF (Fig. 2), a distinctive, cliff-forming, of this deformation, a cross section of the transi- ments suggests that the altered unit consists stratigraphically continuous, siliceous zone tion is exposed along the creek (Fig. 12; see Lo- partly of volcanic rocks. On the basis of the pro- marks the transition between the sheeted dike calized Fold and Thrust Deformation section). gressive upward alteration and silicification of complex and the overlying submarine volcanic From about 100 to 20 m below the siliceous the sheeted dikes, high degree of alteration, and rocks. The volcanic section is devoid of dikes unit, dikes (50%) cut macroscopically highly al- relative abundance of sulfides, the siliceous zone and has a well-defined lower contact with the tered dikes. One dike with recognizable chilled of transition from sheeted dikes to volcanic massive siliceous zone. In contrast, the sheeted margins is progressively altered over a few me- rocks is interpreted to have formed as a result of dike complex has, with increasing alteration, a ters to a rock with chlorite and quartz, the two intense hydrothermal alteration of mafic flows gradational upper contact with the siliceous principle alteration products, segregated from and dikes, concentrated near the site of eruption zone, which is intruded by a few late, irregular, each other. Also some dikes have brecciated ar- on the sea floor. mafic dikes (Fig. 11). Disseminated sulfides are eas with a siliceous matrix. With increasing al- The abruptness of the transition from sheeted common in the siliceous zone, and weathering of teration the sheeted dikes appear to merge up- dikes to volcanic rocks is a key feature of all the

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cross cut by less altered dikes in the sheeted dike NW 200 m SE complex, (2) a Cu-Pb massive sulfide deposit along the transition from dikes to volcanic rocks (Pittsburgh-Tonto mine in Tonto Creek, T, in 3 sheeted dike 2 Fig. 1), (3) the increasing degree of alteration of complex the gabbro toward the base of the sheeted dike 8 complex, suggesting that the degree of alteration 1 4 depended on the depth of fracture-controlled 7 penetration of a hydrothermal system, and (4) 5 the lack of alteration in the Snowstorm Moun- tain gabbro that intruded off the axis of dike in- sd+ trusion and hydrothermal alteration. What dis- 6 tinguishes the transition in the Payson ophiolite from other ophiolites is the high degree of silici- A fication and resulting resistance to erosion to form a distinct map unit.

turbidites Volcanic Section submarine basalt The volcanic section overlying the sheeted dikes is very well exposed in Alder Creek along B altered transition the east flank of the Mazatzal Mountains (Fig. 2). Massive flows with interflow sediments sheeted dikes with screens of: dominate the section; pillowed flows with dis- + qtz diorite tinct selvages are rare. The basalts are generally plagioclase-phyric, similar to the sheeted dikes Larson Spr. fm. below the silicified transition, but finer grained Center Creek and more amygdaloidal. Flow-tops can be rec- Crackerjack Larson Spring granite ognized by large amygdules and breccias with jasper infillings. Clastic interflow sediments in- Figure 12. (A) Cross section (parallel to dikes) of the top of the sheeted dike complex near clude volcanic conglomerates, graywackes, and Center Creek (d in Fig. 2). Note the angular relationship between bedding (diagonal ruled tuffs locally with graded beds and scour-and-fill lines) in screens of the Larson Spring formation (1) and steeply dipping to overturned bedding structures that provide reliable and consistent in the altered transition (2) and overlying volcanic rocks (3). The granitic screens (4) struc- northwest facing directions. Coarse volcanic turally overlie screens of the bedded rocks (1), and both are probably intruded by quartz dior- breccias with a few thin sills occur near the base ite (5). Low-angle faults (6) that cause the altered unit to be more overturned than indicated by of this volcanic section, which is devoid of dikes. bedding are shown schematically. This section of rocks is overturned on the northwest-facing Chemical sediments include lenses of banded limb of a syncline cored by the turbidites (7) and lies in the hanging wall of the east flank thrust iron formation, greenish chert, and massive fault (8). (B) Stereonet showing poles to bedding of the Larson Spring formation in Center jasper. An autobrecciated dacitic flow with Creek (d in Fig. 2) from (A) and same poles to bedding after rotating the volcanic section to jasper infilling is exposed in Alder Creek, and horizontal (arrow). The correspondence of these original orientations to those of the Larson other dacitic breccias occur along strike to the Spring formation in both the Crackerjack (b in Fig. 2) and Larson Spring (c in Fig. 2) roof pen- north and south. Both the lower contact with the dants across the Tertiary valley indicates that most of the plutonic core of the ophiolite was not altered transition and the upper contact with affected by the folding. dacitic flow breccia are exposed, and the lack of folds suggests that the thickness of 400Ð500 m for the basaltic volcanic section in Alder Creek (Fig. 4) is a good estimate. The predominance of best studied ophiolites (Moores, 1982; Pallister, 1992). massive flows and lack of well-defined pillows 1981; Rosencrantz, 1983; Baragar et al. 1987; Silicification and mineralization are concen- is unusual, but the paucity of erupted basalt is Harper, 1984) and distinguishes ophiolites from trated at the transition from sheeted dikes to vol- typical of many ophiolites (usually <1 km; other types of volcanic centers. The lack of dikes canic rocks both in modern (Alt et Moores, 1982) and sections of oceanic crust ex- in even the most well-exposed volcanic section al., 1986; Francheteau et al., 1992) and in other posed on the sea floor (<500 m; Francheteau et is quite remarkable given the high density of ophiolites (e.g., the Josephine ophiolite; Harper al., 1992, and references therein). More magma sheeted dikes (90%Ð100%) less than 100 m et al., 1988) and is indicative of an axial hydro- solidifies within the sheeted dike complex. away. Although the high degree of hydrothermal thermal system active during sea-floor spread- alteration has obliterated the connection between ing. Although the axial hydrothermal system Tonalites and Dacites the dikes and volcanic rocks, the spatial proxim- was most actively corrosive in the top 150 m of ity as well as the textural, geochemical, and iso- the sheeted dike complex of the Payson ophio- In the Payson ophiolite, tonalitic and dacitic topic similarity of the sheeted dikes and volcanic lite, the entire complex was altered during for- rocks are closely associated, both spatially and rocks indicates that they are comagmatic (Dann, mation as indicated by (1) epidosites that are temporally, with all three exposed layers of the

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0 5 10 15 cm B

A 1 meter mafic dike C tonalite dike gabbro

Figure 13. (A) Mafic dikes intruding granophyric tonalite. Of the three mafic dikes, the earliest dike (center) intrudes and mingles with locally unconsolidated tonalite (long arrow) and the two later dikes have more distinct chilled margins (small arrows). (B) Com- posite dike of mafic pillows (black) in tonalite (light gray), which in- trudes gabbro (pattern) (traced from photograph). (C) Composite dike with tonalitic core (pale shades) and mafic margins, which in- trudes mafic dikes (dark shades) with gabbroic screens (pattern) (outcrop sketch). (D) Composite breccia dike with tonalite (pale C shade) intruding the path of an earlier mafic dike (black) in gabbro D (pattern) (traced from photograph). 0 5 10 15 cm

ophiolite pseudostratigraphy—gabbro, sheeted constitute about 10% of the sheeted dike com- tinct suites of tonalite (aphyric and quartz-por- dikes, and volcanic rocks. Rare dacitic flows, plex as a whole (varying between 0% and 38% phyritic) intrude hornblende gabbro near the breccias, and tuffs are interbedded with the ba- locally) and are mutually intrusive with, and par- base of the sheeted dike complex as unusually saltic volcanic rocks. Tonalitic and dacitic dikes allel to, the mafic dikes (see Fig. 10). Two dis-

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thick dikes (≤15 m) and have finer grained equi- may represent this type of composite dike. In ad- odiorite, tonalite, quartz diorite, and possibly valents in the sheeted dike complex. However, dition to the composite dikes, tonalitic dikes potassium feldspar-bearing, quartz gabbro. Pit- both tonalitic rocks and mafic dikes are absent commonly intrude and are intruded by mafic ted surfaces due to the weathering of biotite are from deeper levels of the hornblende gabbro and dikes. Clearly, the tonalitic magma is intimately characteristic of this suite in outcrop. The folia- the Round Valley gabbro, suggesting that they associated with the mafic dikes and the main tion is defined by the alignment of plagioclase are rooted in the high-level gabbro. phase of extensional magmatism. only, not biotite or hornblende, and is considered The most common variety of tonalite forms Most ophiolites contain suites of felsic rocks, to be a magmatic flow foliation. The presence of an extensive sill in the northern part of the map commonly plagiogranites, concentrated in the alkali feldspar, the lower An content of the pla-

area (Fig. 2) along the East Verde River, which upper part of the gabbro below the sheeted dikes gioclase (An35Ð45), much darker hornblende, intrudes gabbro and is intruded by a mafic dike (e.g., Pederson, 1986; Greene, 1989; Saunders et more alteration, more quartz, some local grain swarm. Some early mafic dikes are disrupted by, al., 1979; Pallister and Hopson, 1981). Although size reduction, and easily recognizable and and included in, the tonalite, suggesting that the the tonalite-dacite suite of the Payson ophiolite abundant zircon distinguish this suite of coarse- tonalite was not fully solidified at the time of in- is petrographically similar to, and spatially dis- grained basement rocks from the quartz diorite trusion of the mafic dikes (Fig. 13A). The tona- tributed as, the plagiogranites in other ophiolites, and gabbro of the ophiolite. Some of the most

lite weathers pink and consists of phenocrysts of the suite is more mafic and K2O-rich (Dann, prolific gold mines in the area are hosted by the plagioclase and hornblende needles sparsely dis- 1992). In contrast to the , Oman coarse-grained basement rocks and are associ- persed in a finer grained groundmass, which, in (Pallister and Hopson, 1981), the Payson ophio- ated with quartz veins, trending northwest paral- thin section, contains graphic intergrowths of lite is unusual for the high density of tonalitic lel to, and closely associated with, the mafic dike quartz and altered plagioclase. A distinctive fea- and dacitic dikes in the sheeted dike complex, swarm of the ophiolite. These mineralized veins ture of this suite is the presence of lots of small the great thickness of some tonalitic dikes in the are thought to be Tertiary (Wrucke, et al., 1983), (≈1 cm), subspherical enclaves of more mafic gabbro just below the sheeted dike complex, and but their association with the dikes and absence composition. Unlike xenoliths, the enclaves have its preintrusive to synintrusive timing with the in younger granodiorites (ca. 1.71 Ga) suggests lobate margins, some sharp and others grada- mafic dikes. Mafic magmatic inclusions and the that they may be a product of the ophiolitic hy- tional and are suggestive of mingling between contemporaneous intrusion of the tonalitic and drothermal system (ca. 1.73 Ga). the tonalite and a hybrid magma of mixed basalt mafic magmas suggests that magma mixing may A northeast-trending belt of hypabyssal and tonalite (e.g., Dorais, et al., 1990). Tonalitic have contributed to the more mafic composition isotropic granite underlies the northwest-facing and dacitic dikes with the same enclaves are of the Payson tonalites. The generation of tona- felsic volcanic rocks of the Larson Spring for- commonly the earliest hypabyssal intrusions lite followed intrusion of gabbroic magma into mation and occurs as screens in the sheeted dike into screens of gabbro or granite preserved in the the basement complex, which is locally remobi- complex (Fig. 9) and roof pendants in the gab- sheeted dike complex. The enclaves of more lized as felsic melts and provides a potential bro. Along the East Verde River (a in Fig. 2) the mafic composition, along with the mutually in- source for the tonalite-dacite suite (see y in granite is a pink, fine-grained granophyre with trusive relations between some mafic dikes and Fig. 8B). However, incompatible element deple- miarolitic cavities near its upper contact with the the tonalite, indicate that intrusion of the mafic tion of some of the gabbros suggests a residual felsic section and is cut by as much as 50% and felsic magmas overlapped temporally dur- felsic liquid was filter pressed from the gabbro mafic dikes. Near Larson Spring (c in Fig. 2) the ing development of the ophiolite. (Dann, 1992), similar to the Karmoy ophiolite felsic section is underlain by granodiorite, and The character of composite dikes is deter- (Pederson and Malpas, 1984). together they sit as a roof pendant in the gabbro mined by the relative timing of intrusion of the and are cut by mafic dikes (<20%). The granitic mafic and tonalitic magma along the same path- BASEMENT COMPLEX screens in the sheeted dike complex are usually way. The first of three types consists of a series highly altered, but several fresh samples are of disconnected, lobate pillows of basalt within Coarse-grained granitoids, hypabyssal gran- pink, fine-grained monzogranite. In light of the tonalitic dikes that commonly intrude gabbro ite, and submarine felsic volcaniclastic rocks are volume of intrusion of mafic magma, it is sur- (Fig. 13B). The basaltic pillows have serrated disseminated throughout the Payson ophiolite prising that the contact between the granite and chilled margins that are intruded by dikelets of and are collectively referred to as the basement felsic section is so linear on the map (Fig. 2). tonalite and indicate that the basaltic magma in- complex. A northeast-southwest trending belt of Fine-grained, pink, felsic volcaniclastic and truded and was chilled against cooler, partially coarse-grained, foliated, plutonic rocks (Fig. 2) massive volcanic rocks are exposed in five loca- molten tonalite. In another type of composite occurs as a roof pendant in gabbro, intruded by a tions (Fig. 2). The felsic section overlies fine- dike, a mafic dike is cored by a dacitic or tona- mafic dike swarm and small plutons of por- grained granitoid rocks, and together they are in- litic dike with gradational contacts (Fig. 13C). phyritic diorite. Although the contacts with gab- truded by diorite and gabbro and mafic dikes. Coarser grained composite dikes have a zone of bro are poorly exposed and more complex in de- The formation takes its name from exposures at mixing with mafic phenocrysts from the mafic tail than shown on the map (Fig. 2), isotropic Larson Spring (c in Fig 2) where felsic volcani- magma in a hybrid matrix. In this case, the tona- gabbro truncates the foliation of the granitoids in clastic breccia is overlain by thick beds (5 m), litic magma intruded before the center of the several places, and the relationship is clearly in- graded from breccia to porcellanite, some chert, mafic dike was solidified. A third type of com- trusive. Coarse-grained, quartz monzodiorite has and mafic sediments. Thinly bedded (10 cm) posite dike intruding gabbro consists of a dense a U/Pb zircon age of 1750 ± 3 (Dann et al., plagio-arenites with shallow scour-and-fill struc- breccia of mafic dike material in a tonalitic ma- 1989), 20 Ma older than the ophiolite and one of tures and graded beds are common and consis- trix (Fig. 13D). Apparently, the tonalite intruded the oldest rocks in central Arizona. Small iso- tently indicate the section faces northwest. The along the path of an earlier fully, or almost fully, lated exposures of this unit occur in gabbro from graded beds and chert indicate the sequence was solidified mafic dike. Isolated outcrops of intru- both the northwestern- and southeasternmost deposited in a submarine environment. The sion breccias full of dike and gabbroic xenoliths corners of the map area. This suite includes pink northeasternmost exposure is mostly massive, in a tonalitic matrix are found sporadically and monzogranite, gray quartz monzodiorite, gran- aphyric felsite which may be flows. The Crack-

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erjack Mine roof pendant (b in Fig. 2) is cut by a bound the sheeted dike complex. The abrupt orite is an intrusion breccia of more mafic rock in locally sheeted dike swarm which includes along-strike terminations of the sheeted dike a granodioritic to dioritic matrix. Unfortunately, dacitic dikes. The Larson Spring formation is complex are due to northeast-trending normal Highway 89 covers most of the shear zone, and well exposed north of Gisela (e in Fig. 2) and faults that juxtapose the subhorizontal layer of its relationship to the granodiorite is uncertain. consists mostly of felsic volcaniclastic breccia in sheeted dikes with the underlying gabbro. The Dextral shear along northeast-striking structures southeast-facing graded beds. This section is in- northwest-trending set have physiographic ex- is consistent with later deformation. However, truded by mafic dikes and gabbro and is overlain pression and are locally exposed as fault gauge several outcrop-scale, northeast-trending dextral by submarine mafic volcanic rocks which prob- in propylitic altered rock, cut by veins of calcite. shear zones and faults that are intruded by dikes, ably are part of the ophiolite. Outcrops of nearly horizontal Cambrian Tapeats suggesting the possibility that the strike slip shear A small section of felsic volcaniclastic rocks Formation decrease in elevation by 600 m to the zone may be related to a transform fault, active is poorly exposed as screens within 100 m of the southeast across at least three northwest-trend- during formation of the basin. base of the volcanic section near the top of the ing Tertiary faults (Dann, 1992). The overall ef- sheeted dike complex just north of Center Creek fect is to expose deeper levels of the ophiolite to Localized Fold and Thrust Deformation (d in Fig. 2). Bedding is recognizable in an area the northeast and preserve the sheeted dike com- of lower dike density and has a distinct angular plex by successively dropping it down to the The transition from sheeted dikes to volcanic relationship with bedding in the overlying tran- southwest (Fig. 2). rocks is steeply dipping to overturned on the sition (see Fig. 12). Since the transition from The northwest-trending Tertiary faults are northwest-facing limb of a syncline cored by the sheeted dikes to volcanic rocks corresponds to parallel to the dikes and potential normal faults overlying turbidites, along the east flank of the the pre-ophiolite paleosurface, the Larson that may have formed during sea-floor spread- Mazatzal Mountains (Fig. 2). The ca. 1.70 Ga Spring formation must thin from about 500 m at ing. One fault-bound domain of southwest-dip- unconformity at the base of the Mazatzal Group “a” to near 20 m at “d” (Fig. 2). This thinning ping dikes has been rotated about 30¡ relative to truncates the syncline and cuts down section probably reflects a pre-ophiolite angular uncon- the adjacent domains of northeast-dipping dikes. from the turbidites to the sheeted dikes. Since the formity (see Localized Fold and Thrust Defor- The 5¡Ð6¡ of rotation of nearby exposures of the volcanic section has been eroded off most of the mation section). Cambrian Tapeats formation suggests that Ter- exposed ophiolite that lies east the Tertiary val- tiary extension cannot account for rotation of the ley, no paleohorizontal reference frame is avail- STRUCTURE southwest-dipping domain of dikes. As a result, able to evaluate rotations. The orientations of this rotation may have accompanied offset on steeply dipping, northwest-trending dikes would The present structure of the ophiolite has three listric normal faults active during sea-floor not change during rotations about axes orthog-

major components. First, roof pendants of the spreading. Normal faults, rotated blocks, and onal to the dikes. Therefore, D2 and D3 deforma- basement complex were rotated prior to devel- grabens occur along the Mid-Atlantic Ridge and tions with northeast-trending, subhorizontal fold opment of the ophiolite (ca. 1.73 Ga). The axis in several well-exposed ophiolites. For example, axes would have no obvious affect on the orien- of this rotation (northeast) is orthogonal to the rotated domains of sheeted dikes and volcanic tation of the northwest-trending dikes. However, fold axis of the early compressional deformation rocks define grabens in the Troodos ophiolite correlation of bedding in the basement complex

(D1, ca. 1.74 Ga) of the Ash Creek block (Fig. 1) that formed parallel to the spreading axis (Vargas discussed below suggests that the effects of fold- and, therefore, probably is unrelated to the only and Moores, 1985), and in the Josephine ophio- ing are localized along the east flank, and the known pre-ophiolite tectonic event. Instead, the lite, entire crustal sections are rotated as much as steep dips of the volcanic section do not apply to basement complex may have undergone early 50¡ relative to the overlying sediments and un- the exposed plutonic core of the ophiolite. extensional rotation related to the rifting that led derlying Moho (Harper, 1984). In both cases, the The Larson Spring formation occurs in a small to development of the ophiolite. Second, the rotated dikes are cut by late subvertical dikes, isolated area of screens at the top of the sheeted ophiolitic pseudostratigraphy or primary layer- connecting the rotation to the time of sea-floor dike complex just below the transition to volcanic ing of gabbro, sheeted dikes, and volcanic rocks spreading. Although it is reasonable to expect rocks north of Center Creek (d in Fig. 2). Poorly formed during extension, and several related primary extensional structures in the Payson defined bedding in felsic breccias, tuff, and pla- structures may be preserved. Third, the Mazatzal ophiolite, dikes have yet to be found cutting nor- gio-arenite dips to the southeast and is intruded by

and Yavapai (D2 and D3) and Tertiary mal faults. mafic dikes. The 55¡ angle between the bedding extension produced secondary structures that in the altered transition zone and in the basement offset and deform the ophiolitic pseudostratigra- Strike-Slip Shear Zone screens (Fig. 12A) suggest that the pre-ophiolite phy. Justification for organizing the different paleosurface was an angular unconformity, which rock types of the ophiolite into a pseudostrati- The most prominent fold of the sheeted dike explains why granite is found so close to the pale- graphic sequence rests on knowing the effects of complex occurs in the RC area (Fig. 2). The ori- osurface. In addition, the felsic section appears in later deformation. entation of dikes and flow foliation in the gab- the cross section to be structurally overlain by broic screens changes gradually about 75¡ from granitic screens, suggesting the section of base- Extensional Faults the usual northwest strike to a north-south strike ment rocks is overturned. without any fabric development. Both the sheeted The rotation that restores the bedding in the Tertiary faults bound the sediment-filled val- dikes and a dike swarm in gabbro with prominent transition from sheeted dikes to volcanic rocks to ley that covers the ophiolite and separates the thick tonalitic dikes end abruptly at a poorly de- its original horizontal orientation should also re- western area of volcanic rocks overlying sheeted fined northeast-striking boundary with a younger store the dikes and basement screens to their ori- dikes (EF) from the eastern area of sheeted dikes body of granodiorite (Fig. 2). The dikes are ro- entation at the time of formation of the ophiolite. (RC) overlying gabbro (Fig. 2). The RC area is tated clockwise about a steeply plunging axis, After this rotation is applied, the bedding in the divided by parallel sets of northeast- and north- suggesting proximity to a dextral, strike-slip, basement screens has a northwest dip that west-trending normal faults, some of which shear zone. The northeast margin of the granodi- matches the dip of the bedding in the basement

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complex (roof pendants b and c in Fig. 2) further juvenile magmatic arc rocks ( Nd=4-5) north (Fig. 12B). In addition, the granitic screens 1.74 Ga 1.735 Ga 1.72 Ga 1.71 Ga submarine volcanics- tonalite turbidites with structurally overlying the bedded basement volcaniclastic massive sulfides batholith screens are restored to underlying them as in the sediments ash beds roof pendants (Fig. 12A). The correspondence between the orientation of bedding in the roof pendants and of the restored bedding in the screens suggests that the bedding in the roof pen- dants did not undergo the same rotation that overturned the transition from sheeted dikes to volcanic rocks. Consequently, the plutonic core of the ophiolite east of the Tertiary valley was not affected by the same scale of folding that overturned the volcanic rocks along the east 1.71 Ga 1.75 Ga granodiorite basement complex flank. Whether the folding along the east flank is basalt sheeted dikes localized in a high strain zone and/or representa- Ash Creek block Mazatzal block intra-arc basin ( =2-3) gabbro tive of deformation in the upper, less isotropic Nd 1.73 Ga layers of the crustal section remains speculative. Payson ophiolite The conclusion that the plutonic core of the pull-apart basin ophiolite has not been significantly folded has important implications for the original orienta- tion of magmatic layering. The orientation of magmatic layering of gabbro in the Payson ophi- arc-parallel olite is generally northeast-trending and moder- strike-slip fault system ately to steeply dipping. If the magmatic layering (parallel to regional deformational fabric and convergent plate boundary) was originally subhorizontal, the present steep dips would indicate substantial tectonic rotations. Figure 14. Schematic block diagram of the Mazatzal and Ash Creek crustal blocks at the However, tectonic rotations of this magnitude are time of formation of the Payson ophiolite (ca. 1.73 Ga). Northwest-trending zones of sea-floor not consistent with the map pattern and are con- spreading generated the exposed ophiolitic crustal section consisting of gabbro, sheeted dikes, trary to the earlier conclusion, based on bedding and submarine basalt and containing screens and a roof pendant of older crust (ca. 1.75 Ga). in the basement complex, that the plutonic core The ophiolitic basin floor was covered with felsic breccia and turbidites with air-fall ash beds of the ophiolite has not been significantly rotated. (ca. 1.72 Ga) and andesitic volcanic centers. In contrast, submarine volcanic rocks and sedi- Consequently, the magmatic layering must have ments of the Ash Creek block had already been folded about a northwest-trending axis prior to formed with originally steep dips. In the Oman intrusion of the Cherry tonalite batholith (ca. 1.735 Ga) and deposition of volcaniclastic sedi- ophiolite, Rothery (1983) found that the dike ments (ca. 1.71 Ga). The discrepancy in Nd isotopes and tectonostratigraphic histories (and swarm locally intrudes gabbro with layering crustal profiles, as only one block has been deformed and thickened) of the two crustal blocks nearly parallel to the dikes. After ruling out large suggests that a terrane boundary (black with white stripes) lies in the vicinity of the younger scale tectonic rotations, he concluded that the Moore Gulch shear zone (black). Because juxtaposition of the two terranes occurred prior to dikes penetrated laterally into originally steep convergent deformation (northeast-trending fabric), the terrane boundary (northeast trend- layering formed along the sides of an adjacent ing) may have been a strike-slip fault zone (arc parallel). Arc-parallel extension in a step over magma chamber. Steeply dipping layering in zone, opening of an intra-arc basin with sea-floor spreading, and juxtaposition of the basin high-level gabbros also occurs in the Lokken with a portion of the arc with a distinct earlier tectonostratigraphic history (Ash Creek block) ophiolite (Greene, 1989). Consequently, the ori- could have happened along a strike-slip fault system (shown schematically beneath the block entation of magmatic layering is less useful in diagram), like those running parallel to modern arcs. understanding the structure of the ophiolite than the orientation of the dikes and distribution of components of the ophiolite pseudostratigraphy. (2) the screens and roof pendants of older arc dike complex. Within a 40 m.y. period of arc ORIGIN AND EMPLACEMENT OF THE crust, (3) the overlying arc-derived turbidites magmatism, the Mazatzal block records rifting PAYSON OPHIOLITE with air-fall ash beds and associated andesitic of older arc crust, the opening of an intra-arc volcanic rocks, and (4) the granodiorite plutons basin by sea-floor spreading, deposition of arc A model for the origin and emplacement of that intruded the basinal crust prior to convergent volcanic and volcaniclastic rocks and arc-de- the Payson ophiolite must account for sea-floor deformation. The formation of the Payson ophi- rived turbidites within the basin, and intrusion of spreading above a subduction zone, the presence olite by a process analogous to modern sea-floor arc granitoids into the ophiolitic basin floor. of the basement complex, the juxtaposition of spreading is supported by (1) the sheeted dike Deformation and uplift of the basin, locally ex- the ophiolite with another arc terrane prior to complex, (2) the abrupt transition to a thin sec- posing the ophiolite, occurred during the ca.

convergent deformation, and the orientation of tion of submarine basalt, (3) the rooting of 1.70 Ga Yavapai orogeny (D2) when the entire arc the sheeted dikes relative to the convergent plate sheeted dikes in coeval gabbro, (4) the distribu- was accreted to North America. The Payson margin. An arc setting is indicated by (1) the tion of synmagmatic hydrothermal alteration, ophiolite intruded and erupted upon the oldest supra-subduction zone geochemical signature, and (5) possible block rotation of the sheeted crustal rocks yet recognized in central Arizona.

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This fact, along with the lack of recognized low- of the Payson ophiolite and/or its juxtaposition crustal section beneath the Marinduque basin has angle thrusts and melange, suggests that no part with the Ash Creek block. an ophiolitic structure with screens and roof pen- of it was obducted as a thrust slice over older How the axis of sea-floor spreading is situated dants of older arc crust similar to the Payson crust. The overall subhorizontal orientation of the on the overriding plate within an arc depends on ophiolite. Continued arc-parallel strike-slip ophiolite pseudostratigraphy, of enveloping sur- the relative motions of plates converging at a movement will juxtapose the intra-arc basin with faces related to upright folding, and of the subduction zone (Dewey, 1980). The Payson arc crust that may have formed hundreds of kilo- 1.70 Ga unconformity suggests that the Mazatzal ophiolite shares many features described from meters away in the same (Sarewitz and block was deformed and uplifted as a coherent the Rocas Verdes ophiolite (Chile), Lewis, 1991) and that may record a quite differ- block within the accreting arc. Unlike ophiolites but the relative orientations of the spreading axes ent tectonostratigraphic history (like the Ash dismembered in zones, which may repre- and convergent margins requires different tec- Creek block). Importantly, this juxtaposition will sent the obducted remnants of ocean basins sub- tonic settings. Similar to the Payson ophiolite, occur prior to regional convergent deformation, ducted prior to collision (), the the Rocas Verdes is exposed as a shallow-dip- and the spreading axis will be orthogonal to the Payson ophiolite developed in situ as a distinct ping pseudostratigraphic sequence of gabbro, fabric of future convergent deformation of the arc extensional phase during the complex evolution sheeted dikes, and submarine basalts overlain by (barring any large rotations relative to the plate of an Early Proterozoic arc. arc-derived turbidites (de Wit and Stern, 1981). boundary)—both key features of how the Payson Although the Payson ophiolite developed in The gabbro includes fragments of older conti- ophiolite is situated within the Early Proterozoic situ relative to the arc crust it intrudes, a sche- nental crust, and the ophiolite is inboard of a orogenic belt. The collage of distinct terranes matic view of the Mazatzal and Ash Creek crus- coeval magmatic arc. The sheeted dikes and the composing the Philippine arc includes fragments tal blocks (during formation of the intra-arc deformational fabric strike parallel to the conti- of older deformed continental and arc crust and basin and prior to 1.70 Ga regional deformation) nental margin, and the model for its origin and several ophiolites, juxtaposed along major struc- reveals their incompatible tectonostratigraphic emplacement involves opening and closing of a tures, locally involving more than a 1000 km of histories (Fig. 14). Note that the Ash Creek block back-arc basin for 600 km along the west coast strike-slip movement, suggesting a general records submarine arc volcanism and northwest- of South America. In contrast, the sheeted dikes model of basin formation and ophiolite emplace-

trending deformation (D1) prior to intrusion of (northwest trending) of the Payson ophiolite are ment along arc-parallel strike-slip faults (Sare- an arc batholith (Karlstrom and Bowring, 1991) nearly orthogonal to both the deformational fab- witz and Lewis, 1991; and references therein). with no evidence of later rifting (i.e., northeast- ric and proposed terrane boundaries (northeast The Payson ophiolite may represent the most an- trending dikes). The basement complex (ca. trending), suggesting that extension and basin cient example of this style of ophiolite generation 1.75 Ga) intruded by the ophiolite is older than opening was parallel to the arc. This geometry is and emplacement and a testimony to the complex rocks of the Ash Creek block but does not record more consistent with an intra-arc basin formed evolution of an Early Proterozoic arc. this early deformational event. The juvenile Nd as a pull-apart structure along a system of arc- isotopic signatures of the Ash Creek block con- parallel strike-slip faults (northeast trending) as CONCLUSIONS trasts with the evidence for a LREE enriched opposed to a back-arc basin formed by roll-back component in all the rocks of the Mazatzal block of the subducting slab. In addition, the strike-slip The Payson ophiolite is exposed as a shallow- (Dann et al., 1993). The crustally thickened model readily explains the paradox of the juxta- dipping, pseudostratigraphic sequence of gab- block would have a different crustal profile than position of contrasting crustal blocks prior to bro, sheeted dikes, and submarine basalts. The the rifted block. These discrepancies indicate the convergent deformation, and therefore not re- distribution of synmagmatic hydrothermal alter- presence of a major structural break or terrane corded by the late, block-bounding shear zones. ation and mingling and mutual intrusion of boundary (ca. 1.73 Ga), subparallel to the The trace of the earlier strike-slip faults may (1) gabbro and finer grained mafic dikes and younger Moore Gulch shear zone (< 1.70 Ga). have been preserved by the juxtaposition of ter- (2) tonalitic and mafic magmas indicates that all Prior to recognition of the ophiolite, older base- ranes with distinct crustal profiles and reacti- the rock types assigned to the ophiolite formed at ment complex, and many age determinations,An- vated during convergent deformation and the same time by a process analogous to modern

derson (1989) cited the overlap of deformed (D2) postassembly differential uplift. In the tectonic sea-floor spreading. A model of an intra-arc sedimentary sequences as evidence that the sub- development of the eastern Indonesian collision basin (Fig. 14) formed along an arc parallel marine volcanic sequences of the Mazatzal block zone, the final position of crustal blocks is con- strike-slip fault is proposed to account for the were deposited on the eastern flank of the older trolled by large scale translation along strike-slip orientation of the sheeted dikes relative to re- sequence (Ash Creek block) and not juxtaposed faults, but because strike-slip faults are inher- gional deformational fabric (convergent margin), along a suture zone during convergent deforma- ently difficult to recognize, small amounts of late the juxtaposition of the distinct terranes prior to tion. The discrepancy in tectonostratigraphic his- thrust faulting will dominate the geologic struc- convergent deformation, and the presence of tories, along with the proposed overlap (by suc- ture (McCaffrey and Abers, 1991). older arc crust as screens and roof pendants cessor basin?), suggests that any major movement The best actualistic model for the Payson within the ophiolite. Sea-floor spreading in this between the terranes would have to have occurred ophiolite may be the Marinduque intra-arc basin particular setting may account for some of the prior to deposition (ca. 1.71 Ga) and convergent in the Philippines, which developed from a pull- unusual features of the Payson ophiolite includ- deformation (ca. 1.70 Ga). The only evidence for apart structure to a sea-floor spreading system ing (1) the thickness and locally dense spacing of deformation as old as the ophiolite near the shear along the arc-parallel Philippine strike-slip fault tonalitic dikes, (2) the off-axis intrusion of gab- zone occurs as subhorizontal synmagmatic zone (Sarewitz and Lewis, 1991). This deep bro, (3) the high degree of alteration of the tran- stretching lineations in a northeast-trending mafic (1.6 km) intra-arc basin (75 by 38 km) is bound sition from sheeted dikes to submarine basalt, dike swarm in gabbro intruding the ca. 1.735 Ga by faults, contains a fossil axial spreading center and (4) the threefold increase of dike width with batholith (Dann, unpub. mapping), which may in- (orthogonal to the strike-slip faults) and some depth in the sheeted dike complex. Another directly reflect the presence of a northeast-trend- submarine volcanoes, and is filling with tur- unique feature of the Payson ophiolite is the an- ing strike-slip boundary active during formation bidites. It is reasonable to speculate that the gular unconformity preserved in the screens and

364 Geological Society of America Bulletin, March 1997

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