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DEFORMATIONAL HISTORY OF THE GRANJENO SCHIST NEAR CIUDAD

VICTORIA, MEXICO

A thesis presented to

the faculty of

the College of Arts and Sciences of Ohio University

In partial fulfillment

of the requirements for the degree

Master of Science

David S. Dowe

June 2004

This thesis entitled

DEFORMATIONAL HISTORY OF THE GRANJENO SCHIST NEAR CIUDAD

VICTORIA, MEXICO

BY

DAVID S. DOWE

has been approved for

the Department of Geological Sciences and

the College of Arts and Sciences by

R. Damian Nance

Professor of Geological Sciences

Leslie A. Flemming

Dean, College of Arts and Sciences

Dowe, David S. M.S. June 2004. Geological Sciences

Deformational History of the Granjeno Schist Near Ciudad Victoria, Mexico (pp. 109)

Director of Thesis: R. Damian Nance

Exposed in the core of a NNW-trending frontal anticline of the Laramide fold-thrust belt of northeastern Mexico, the Paleozoic Granjeno Schist comprises a polydeformed assemblage of metasedimentary and metavolcaniclastic rocks and serpentinized mafic- ultramafic units. Deformation of the Granjeno Schist has produced at least four sets of structures. The earliest deformation (D1) predates emplacement of a leucogranite at

351±54 Ma and may record obduction of this oceanic unit. Subsequent deformations

(D2-D4) record the tectonic juxtapositioning of the Granjeno Schist against the Grenville-

aged Novillo Gneiss by NNW-directed dextral shear under conditions of decreasing

temperature. Mica cooling ages of 313±13 Ma and 300±4 Ma are considered to date the

onset of dextral motion, which continued into the Permian. These events are linked to the

Late Paleozoic closing of the Rheic Ocean.

Approved: R. Damian Nance

Professor of Geological Sciences

Acknowledgements

I would like to thank the members of my advising committee – Drs. Damian Nance, Greg

Nadon and David Schneider, for without their encouragement, advice, and revisions, this project could not have been completed. More thanks to Damian Nance for providing the opportunity to work in Mexico, and to Drs. Duncan Keppie and Fernando Ortega-

Gutiérrez for their keen ideas and geological knowledge that they willingly shared in the field. Many thanks to Victoria Tong, for her vast technical knowledge that helped carry this thesis through many hardships. A great thank you to my wife, Loretta Ransom, whose time, energy, and personal sacrifices, have ultimately made this thesis a completed project. 5 Table of Contents

Page Abstract...... 3 Acknowledgements...... 4 List of Tables ...... 7 List of Figures...... 8 List of Plates ...... 9 I. Introduction ...... 11 II. Regional Setting ...... 15 Geology of Mexico ...... 15 Guachichil Terrane...... 15 Huizachal-Peregrina Anticlinorium...... 18 Stratigraphy...... 18 III. Field Relations and Petrography...... 25 Introduction...... 25 Granjeno Schist...... 26 Metasedimentary units...... 26 Pelitic Schist...... 26 Interbedded Pelitic and Psammitic Schist...... 33 Metachert ...... 35 Graphitic Schist...... 35 Metavolcaniclastic Schist...... 37 Metaigneous Units ...... 45 Metabasalt...... 45 Serpentinite and Metagabbro...... 47 Serpentinite ...... 47 Metagabbro ...... 49 IV. Structural Geology...... 52 Introduction...... 52 D1 Structures...... 52 D2 Structures...... 61 D3 Structures...... 66 D4 Structures...... 69 Structural Analysis...... 71 Relationship to faulting...... 75 V. Geochronology...... 78 VI. Deformation and Metamorphism...... 83 Introduction...... 83 D1 Tectonothermal Event...... 83 D2 Tectonothermal Event...... 85 D3 Tectonothermal Event...... 91 D4 Tectonothermal Event...... 92 VII. Tectonic History of the Granjeno Schist ...... 94 VIII. Potential linkages to the Acatlán Complex of southern Mexico...... 97 6 References...... 101 Appendix...... 109

7 List of Tables

Table 5.1 U-Pb data for zircon in leucogranite ...... 80 Table 5.2 40Ar/39Ar data for (a) muscovite in leucogranite and (b) phengite in quartz keratophyre ...... 82

8 List of Figures

Figure 1.1 Location of the Huizachal-Peregrina Anticlinorium ...... 12 Figure 1.2 General geology of the Huizachal-Peregrina Anticlinorium...... 13 Figure 2.1 The tectonostratigraphic terranes of Mexico...... 16 Figure 2.2 The Guachichil Terrane of northeastern Mexico...... 17 Figure 2.3 Generalized stratigraphic column of rocks that overlie the Novillo Gneiss within the Huizachal-Peregrina Anticlinorium...... 21 Figure 2.4 Timing of deformations in sedimentary rocks overlying the Novillo ...... 23 Figure 4.1 D1/D2 in Subarea 1...... 53 Figure 4.2 D3 in Subarea 1...... 53 Figure 4.3 D4 type locality in Subarea 1 ...... 54 Figure 4.4 Structural relationship in Subarea 1 on limbs and hinges of F1/F2...... 55 Figure 4.5 D1/D2 in Subarea 2...... 56 Figure 4.6 D3 in Subarea 2...... 56 Figure 4.7 Structural relations in Subarea 3...... 57 Figure 4.8 Structural relations in Subarea 4...... 57 Figure 4.9 Major dextral faults in map area...... 58 Figure 4.10 Major sinistral faults in map area ...... 59 Figure 4.11 Average sinistral and dextral fault orientations within the Granjeno Schist, and F4 fold axes...... 59 Figure 4.12 Geology map of the field area showing structural subareas...... 60 Figure 4.13 Diagram used to estimate the amount of shortening associated with the D3 crenulation of the S1/S2 fabric...... 67 Figure 4.14 The dip isogons for F4 fold classification...... 73 Figure 4.15 Changes in layer thickness across the F4 fold...... 74 Figure 5.1 U-Pb concordia diagram for zircons from the leucogranite ...... 79 Figure 5.2 40Ar/39Ar apparent age spectra of muscovite in leucogranite and phengite in quartz keratophyre ...... 81 Figure 6.1 Temperature-time diagram for the Granjeno Schist showing mineral parageneses in relation to deformational phases and geochronological data ...... 84 Figure 8.1 Pangea reconstruction showing location of Granjeno Schist and Permo- Triassic arc...... 100

9 List of Plates

Plate 3.1 Typical pelitic schist...... 27 Plate 3.2 Photomicrograph of an F1 or F2 isoclinal fold...... 29 Plate 3.3 Photomicrograph of pelitic schist showing main foliation...... 29 Plate 3.4 Photomicrograph of main foliation in pelitic schist showing foliation-parallel chloritic lenses ...... 30 Plate 3.5 Photomicrograph of pelitic schist showing albite porphyroblast (upper center) preserving linear to curvilinear graphite inclusions...... 32 Plate 3.6 Interbedded pelitic and psammitic schist showing cm-scale alternation of mica- and quartz-rich layers...... 34 Plate 3.7 Photomicrograph of interbedded pelitic and psammitic schist showing layers of pelite and psammite ...... 34 Plate 3.8 Photomicrograph of retrogressed garnet porphyroblast within a psammitic layer of interbedded pelitic and psammitic schist...... 36 Plate 3.9 Hand sample of metachert showing interbedded microcrystalline and siliceous- micaceous layers...... 36 Plate 3.10 Outcrop of graphitic schist showing intensely developed structural fabrics reflecting its high graphite content ...... 38 Plate 3.11 Photomicrograph of graphitic schist showing a high modal abundance of graphite and quartz...... 38 Plate 3.12 Outcrop of mafic volcaniclastic ...... 39 Plate 3.13 Volcaniclastic schist with light blue-green and dark gray layers...... 40 Plate 3.14 Photomicrograph of a plagioclase porphyroblast in mafic volcaniclastic at extinction...... 42 Plate 3.15 Photomicrograph of clinozoisite porphyroblast at high angle to the foliation 44 Plate 3.16 Hand specimen of metabasalt showing more massive (less foliated) appearance relative to other schist lithologies...... 46 Plate 3.17 Photomicrograph of metabasalt showing a weak foliation ...... 46 Plate 3.18 Pale-green serpentinite in tectonic contact with the Granjeno Schist ...... 48 Plate 3.19 Metagabbro boulder with hypidiomorphic-granular texture consisting largely of retrogressed plagioclase and hornblende...... 50 Plate 4.1 Photomicrograph of an albite porphyroblast in pelitic schist...... 62 Plate 4.2 Photomicrograph of rootless isoclinal folds...... 62 Plate 4.3 F2 isoclinal fold ...... 64 Plate 4.4 Outcrop of graphitic schist...... 64 Plate 4.5 Photomicrograph of the F2 closure shown in Plate 4.4 ...... 65 Plate 4.6 Small-scale F3 folds in metapelite...... 67 Plate 4.7 F3 hinge zone in metapelite ...... 68 Plate 4.8 S3 in mafic metavolcaniclastic ...... 68 Plate 4.9 Mesoscopic F4 folds...... 70 Plate 4.10 Profile of F4 fold used for its geometric classification...... 72 Plate 4.11 Ductile shear zone separating the Granjeno Schist from the leucogranite...... 76 Plate 4.12 Major sinistral fault in south tributary separating dark Granjeno Schist from metagabbro...... 76 10 Plate 4.13 Ductile shear zone in serpentinite ...... 77 Plate 6.1 S2-C fabric within dextral shear zone separating the Granjeno Schist from the 354±54 Ma leucogranite...... 86 Plate 6.2 “Clast” resembling strain ellipsoid...... 88

11 I. Introduction

In the vicinity of Ciudad Victoria in northern Mexico, the Huizachal-Peregrina

Anticlinorium (Figs. 1.1 & 1.2) exposes metamorphosed Paleozoic (Granjeno Schist) and

Precambrian (Novillo Gneiss) rocks potentially correlative with those of the

Appalachian-Ouachita and Grenville orogens, respectively (Ortega-Gutierrez, 1978,

Ramírez-Ramírez, 1992). Although such a correlation would have profound implications for late Precambrian-Paleozoic continental reconstructions, the nature of the linkages is uncertain because critical aspects of the geology of the anticlinorium are unknown. The

Grenville-aged (1 Ga) rocks of the Novillo Gneiss may also be linked to those of the

Grenville-aged Oaxacan Complex in southeastern Mexico, which are thought to have been sutured to Laurentia following the closure of either the Iapetus (Ortega-Gutiérrez et al., 1999) or Rheic (Keppie and Ramos, 1999) Oceans. Like the gneisses of the Oaxacan

Complex, the Novillo Gneiss is unconformably overlain by Lower Paleozoic sedimentary strata that contain fauna of Gondwanan affinity (Robison and Pantoja-Alor, 1968;

Stewart et al., 1999), suggesting they represent portions of the Gondwanan margin of these oceans (Yanez et al., 1991; Ortega-Gutiérrez et al., 1999). By contrast, Lower

Paleozoic strata of Laurentian affinity unconformably overlie Grenville basement rocks of the Appalachian Orogen.

Strong similarities also exist between the Acatlán Complex, which borders the

Oaxacan Complex, and the Granjeno Schist of the Huizachal-Peregrina Anticlinorium.

Both assemblages are Paleozoic in age, both are metamorphosed and polydeformed, and both contain mafic-ultramafic rocks of possible ophiolitic origin. In addition, both are juxtaposed against rock of Grenville age and have been interpreted to comprise deposits 12

Figure 1.1 Location of the Huizachal-Peregrina Anticlinorium (star) in relation to major cities in Mexico. The Acatlán and Oaxaca Complexes of southern Mexico may have Paleozoic linkages to the study area. 13

Figure 1.2 General geology of the Huizachal-Peregrina Anticlinorium. The core of the anticlinorium exposes pre-Mesozoic units, with poorly understood histories and interrelationships. Map modified from Ramírez-Ramírez (1992). Cross-section through field area modified from Carillo-Bravo (1961).

14 of an ocean floor-accretionary prism (Ramírez-Ramírez, 1992; Ortega-Gutiérrez et al.,

1999). Recent work on the Acatlán Complex (Ortega-Gutiérrez et al., 1999; Malone et al., 2000, Nance et al., 2004) has revealed tectonothermal pulses that have been used to support linkages to the Appalachian Orogen. Hence, clarification of linkages between the

Granjeno Schist and the Acatlán Complex would aid in the development of tectonic models for the Paleozoic evolution of Mexico. Existing models for the evolution of the

Huizachal-Peregrina Anticlinorium are primarily based on radiometric dating and lithological investigations. The present study, a structural-kinematic analysis of the southwestern block of the Granjeno Schist within the anticlinorium, is being used to test and modify these models on the basis of presently unavailable tectonic constraints. 15 II. Regional Setting

Geology of Mexico

The geology of Mexico is of particular interest since an understanding of its tectonostratigraphy and geological evolution remains in its infancy. Economically,

Mexico is rich in mineral resources, and the distribution of these resources appears to be controlled largely by basement geology (Campa and Coney, 1983). Since a substantial proportion of Mexico is covered by Cenozoic volcanic rocks, detailed geologic investigations are a necessity to the complete understanding of Mexico’s role in continental reconstructions and distribution of mineral resources.

Mexico has been subdivided into 16 tectonostratigraphic terranes, the boundaries between which are mostly inferred from sparse pre-Jurassic outcrops, geophysical and isotopic data (Sedlock et al., 1993). The terranes presently are part of the North

American plate, yet their tectonic histories have varied significantly in the past. The majority of the terranes of eastern Mexico were accreted to Laurentia in the Devonian, and later displaced in the Permo-Carboniferous as southern Laurentia collided with northern South America during the formation of Pangea (Keppie and Ortega, 1995). The terranes of southwestern Mexico were derived from the Pacific region between the mid-

Mesozoic and early Cenozoic (Figure 2.1).

Guachichil Terrane

The Guachichil terrane of northeastern Mexico (Figure 2.2) contains Grenville- aged basement overlain by pre-Permian Paleozoic rocks that occupy fault-bounded blocks. Turbiditic Middle Silurian strata rest directly on this basement and are 16

Figure 2.1 The tectonostratigraphic terranes of Mexico. Chatino (CH); Chortis (CHO); Coahuiltecano (C); Cochimí (COC); Cuicateco (CU); Guachichil (GU); Maya (M); Mixteco (MIX); Nahuatl (N); Pericú (P); Seri (S); Tahue (TA); Tarahumara (TAR); Tepehuano (TE); Yuma (Y); Zapoteco (Z). Star within the Guachichil terrane is the approximate location of the Huizachal-Peregrina Anticlinorium. Modified from Sedlock et al. (1993). 17

Figure 2.2 The Guachichil Terrane of northeastern Mexico consists of pre-Mesozoic units that acted as one unit as late as the mid-Jurassic, given that this is the age of the oldest unconformably overlying unit. Rare exposures of pre-Mesozoic rocks, have variably been interpreted as Laurentian derivatives, or transferred from the Gondwanan margin during the formation of Pangea. Modified from Sedlock et al. (1993). 18 unconformably overlain by Lower Pennsylvanian platform strata. Overlying Lower

Permian volcanogenic flysch deposits are extensive, younger Mesozoic cover rocks that deny access to a majority of the pre-Mesozoic rock. Tectonically juxtaposed against these pre-Mesozoic rocks is a Paleozoic assemblage known as the Granjeno Schist, which comprises metabasites, metasediments, and serpentinite-metagabbro bodies. The

Mesozoic succession, which unconformably overlies all Paleozoic rocks, comprises

Lower Jurassic redbeds and evaporites, and Upper Jurassic carbonates and fine-grained clastic rocks. The unmetamorphosed Paleozoic succession has been interpreted to represent a miogeoclinal sequence, whereas the Granjeno Schist is thought to be subduction-related (Stewart et al., 1993). The combination is thought to record a basinal setting near the southern margin of North America, the closure of which juxtaposed the two assemblages (Sedlock et al., 1993). The Mesozoic rocks, on the other hand, represent a post-collisional, rift-drift succession related to the breakup of Pangea (Stewart et al., 1993). Latest Cretaceous to mid-Eocene east- to northeast-vergent thin-skinned thrusting (Laramide deformation) affects the Late Jurassic to Cretaceous cover strata and occurred as a result of an eastward-facing subduction zone on the Pacific margin

(Sedlock et al., 1993).

Huizachal-Peregrina Anticlinorium

Stratigraphy

The pioneering work of Ramírez-Ramírez (1974, 1992) in the Huizachal-

Peregrina Anticlinorium has provided a basic geological knowledge of the metamorphic rocks that crop out in this region. The Huizachal-Peregrina Anticlinorium is located in the Guachichil terrane of Sedlock et al. (1993), also known as the Eastern Sierra Madre 19 terrane (Dickinson and Lawton, 2001), and exposes metamorphosed and deformed

Paleozoic (Granjeno Schist) and Precambrian (Novillo Gneiss) rocks in its core as well as deformed, but unmetamorphosed, Paleozoic strata.

The Novillo Gneiss consists of a variety of Grenville-aged rocks including gabbro-anorthosite, granite, amphibolite, metaquartzite, talc-silicate units, and marble

(Ramírez-Ramírez, 1992). U-Pb geochronology (Cameron et al., 2004) reveals two groups of meta-igneous protoliths, an older assemblage emplaced between ca. 1235 Ma and ca. 1115 Ma, and a younger anorthosite-mangerite-charnockite-granite (AMCG) suite emplaced between ca. 1035 Ma and ca. 1010 Ma. Emplacement of a post-tectonic anorthositic pegmatite at 978±13 Ma followed granulite facies metamorphism dated at

990±5 Ma. The Novillo Gneiss is in fault contact with the Granjeno Schist, it is in fault contact and intruded by a leucogranite body, and, in the northeast portion of the anticlinorium, it is unconformably overlain by Silurian strata of Gondwanan affinity. The deformed, yet unmetamorphosed, pre-Mesozoic strata overlying the Novillo Gneiss include Silurian and Mississippian to Early Permian units (Stewart et al., 1999).

The Granjeno Schist consists of metasedimentary units rich in mica and graphite, metavolcanic and metavolcaniclastic units with mafic signatures, and tectonic slivers of serpentinite-metagabbro (Ramírez-Ramírez, 1992). The Granjeno Schist lies in fault contact with all pre-Mesozoic lithologies (Fig. 1.2) and is unconformably overlain by

Lower Jurassic redbeds (Ramírez-Ramírez, 1992). The following describes the geological history of the various units of the anticlinorium, compiled from several authors. 20 Portions of the unmetamorphosed Paleozoic strata within the anticlinorium have been described by Stewart et al. (1999), from which the following interpretation of their geological histories is derived. Based on faunal analysis (Boucot et al., 1997) and similarities with fauna of Venezuela (Stewart et al., 1999), the strata that unconformably overlie the Novillo Gneiss in the northeast corner of the anticlinorium are Silurian (Early to mid-Wenlock. 430-424 Ma; Okulitch, 2003) and of Gondwanan affinity. The Silurian strata have been interpreted as shallow-water photic zone marine deposits, with the presence of volcanic detritus suggesting nearby volcanic activity (Stewart et al., 1999)

(Figure 2.3).

Erosion or nondeposition is inferred for the Devonian since the post-Devonian strata that immediately overlie the Silurian units appear to be conformable, although exposures are poor (Stewart et al., 1999). Faunal analysis and sedimentological interpretation suggests deposition in a shallow marine environment in proximity to the

North American craton during the lower Mississippian (Early Osagean, 351-342 Ma;

Okulitch, 2003). Overlying this marine succession is a rhyolite unit; a Mississippian age which is indicated by a U-Pb zircon lower intercept of 334±39 Ma, and by the presence of rhyolite clasts in overlying Pennsylvanian units. The upper intercept age of 1086±94

Ma obtained from the rhyolite is statistically identical with the Novillo Gneiss and lies within the range of Grenville-aged Mexican basement. Deformation of the Paleozoic strata prior to the deposition of deep-water marine turbidites of the Lower to mid

Pennsylvanian (314-306 Ma; Okulitch, 2003), which unconformably 21

Figure 2.3 Generalized stratigraphic column of rocks that overlie the Novillo Gneiss within the Huizachal-Peregrina Anticlinorium (Stewart et al., 1999). Silurian strata unconformably overlying the Novillo Gneiss contain fossils of Gondwanan affinity, whereas younger units (Lower Mississippian Vicente Guerrero Formation) suggest proximity to Laurentia. The age and faunal affinities of the sedimentary succession indicate that the Novillo Gneiss and its sedimentary cover had transferred from a Gondwanan margin to Laurentia by the Lower Carboniferous. 22 overlie the lower units, together with igneous activity and faunal provenance interpretation, suggest the approach of the Ciudad Victoria area to Laurentia during waning stages of the Appalachian-Ouachita Orogeny (Stewart et al., 1999).

An Early Permian unit, the age of which is based on faunal analysis, is everywhere interpreted to be in fault contact with older units, thereby precluding recognition of its direct depositional relationship with older strata. The presence of widespread fine-grained distal turbidite layers, as well as Bouma divisions and volcaniclastic sandstone have been used to interpret the unit as a deep-water flysch deposit, potentially coeval with volcanic activity (Stewart et al., 1999).

Unconformably overlying all Paleozoic and older units are coarse Lower Jurassic conglomerates (Fig. 1.2) overlain by thick clastic and carbonate rocks. This combination suggests a rift-drift succession associated with the opening of the Gulf of Mexico and the

Atlantic Ocean (Sedlock et al., 1993).

The unconformable relationships between various unmetamorphosed Mesozoic and Paleozoic units and the Novillo Gneiss, may be used to constrain the ages for the multiple phases of Phanerozoic deformation recorded by these units. At least two periods of deformation occurred in the Paleozoic to earliest Mesozoic (Figure 2.4). The first is confined to the interval Lower Mississippian to Early-mid Pennsylvanian since an angular unconformity separates units of these ages. A second period of deformation is similarly constrained between the Lower Permian and the Lower Jurassic. Further deformation and thin-skinned Cretaceous to Early Tertiary Laramide thrusting are largely confined to Mesozoic strata, however, the effects of this deformation in pre-Mesozoic units, if any, are uncertain. 23

Figure 2.4 Timing of deformations (a minimum of two periods) recorded in sedimentary rocks overlying the Novillo Gneiss as indicated by angular unconformities and the ages of the units separated by these erosional boundaries. 24 The nature and timing of deformational events in the Granjeno Schist have largely been ignored, and its relationship to those in adjoining and surrounding units is controversial. The following chapter provides details of the Paleozoic metamorphic rocks of the Huizachal-Peregrina Anticlinorium, namely the Granjeno Schist. 25 III. Field Relations and Petrography

Introduction

The Granjeno Schist was mapped in the Cañon de Novillo, which extends through part of the Huizachal-Peregrina Anticlinorium, in an attempt to decipher its deformational history (Fig. 1.2). The field area is bound on the east by the Novillo River, to the north and south by the east-west trending canyon, and to the west by the west limb of the anticlinorium. The schist was examined mainly in river sections where exposures were plentiful, as well as in other exposures along road cuts. Grenville-aged gneisses within the canyon were also mapped and are the topic of MS thesis research of Robert Trainor.

Detailed structural investigations of the Granjeno Schist included measurement of bedding, foliations, fold axes, faults and shear zones, and examination of the interrelationship of these structural features. Measurements were made with a Nexus compass, and approximately 30 oriented hand samples were collected for petrographic and microstructural analysis.

Descriptions of the outcrops in the field area are divided into two broad categories: (1) those exposing the Granjeno Schist (metasedimentary units and interbedded metaigneous lithologies), and (2) those exposing the serpentinite body and its associated metagabbro. These categories are chosen to reflect the different environments in which the two assemblages developed, regardless of the deformational history that these units have shared. 26 Granjeno Schist

Metasedimentary units

Pelitic Schist

In outcrop, the pelitic lithologies are gray to gray-black in color, and often have

very well developed structural fabrics and a phyllitic sheen on their foliation surfaces due

to the abundance of phyllosilicates. On foliation surfaces, an intersection lineation is

often present, defined by the intersection of the main composite foliation and a later

crenulation cleavage. This lineation generally parallels the minor fold axes associated

with the crenulation cleavage. Bedding-parallel quartz veins are also common

throughout the pelitic units, and, in part, provide a record of the various phases of

deformation. The pelitic schists show variable phyllitic to schistose textures, degree of

quartz-mica separation, and porphyroblast development (Plate 3.1).

Depositional and fault-bounded contacts with other metasedimentary lithologies

occur throughout the schist, but contacts with the serpentinite-gabbro body on the

western flank of the field area are invariably tectonic. Pelitic schist is also in fault

contact with leucogranite and the Novillo Gneiss on the eastern flank of the field area.

Internally, the pelitic schist is in fault contact with metabasite and is by far the most

dominant component of the Granjeno Schist, occurring in outcrops that range from a few

meters to more than one hundred meters across. 27

Plate 3.1 Typical pelitic schist consists of near equal proportions of micas to quartz. Bedding-parallel quartz veins form rootless isoclinal folds whose orientations provide a record of the various phases of deformation. 28 In thin section, the pelitic schists are dominated by quartz, white mica (phengite), chlorite, graphite, biotite, plagioclase (albite/oligoclase), and trace amounts of zircon.

Quartz grains are stretched and flattened parallel to the dominant foliation and display undulose extinction, but dynamic recrystallization and/or partial annealing has occurred, as a result of which some quartz grains are nearly equidimensional. Quartz occupies 20 –

40% of the mode. Bedding-parallel quartz veins, which make up 1-10% of outcrops, also consist of partially annealed mosaics of sutured grains. Vein quartz grains have undulose extinction with minor flattening either parallel to the main foliation or parallel to a later crenulation cleavage. Their grain sizes, however tend to be somewhat larger than matrix quartz grains, suggesting that metamorphism was not simply static but, in part, coeval with their deformation (Plate 3.2).

The well-developed foliation is defined by the alignment of phyllosilicates and often shows small-scale deformation (Plate 3.3). The most abundant mica is phengite, which occurs as weakly pleochroic green tabular crystals that account for 20-40% of the mode. In contact with all other major minerals, phengite, along with chlorite, define the bedding-parallel foliation (Plate 3.4).

Chlorite is the next most abundant phyllosilicate, comprising 5-10% of the mode.

It occurs as pale green, pleochroic grains and has low blue to gray-brown birefringence.

Like phengite, chlorite is in contact with all other major minerals and is typically aligned with phengite. Occasional chloritic lenses within the pelitic units lie parallel to the main foliation, but contain chlorite grains whose c-axes lie at high angles to the external foliation (Plate 3.4). 29

Plate 3.2 Photomicrograph of an F1 or F2 isoclinal fold, the axial plane of which parallels the main foliation. Recrystallization following folding has produced a weak granoblastic texture.

Plate 3.3 Photomicrograph of pelitic schist showing main foliation defined by phengite (M), graphite (G), quartz (Q) and chlorite (Ch). Small-scale, intrafolial, isoclinal folding (S1 crenulated by S2) is common and is attributable to the first phase of deformation. 30

Plate 3.4 Photomicrograph of main foliation in pelitic schist showing foliation-parallel chloritic lenses in which individual c-axes lie at high angles to the external foliation. 31 Biotite occurs as a relict fabric-forming mineral, but accounts for much less than

1% of the mode in most metasediments. At one locality, where it was found to co-occur with garnet (almandine), biotite amounts to 1-2% of the mode. In all cases, biotite is largely retrogressed to chlorite, but its existence provides evidence for an episode of higher-grade metamorphism than that recognizable in neighboring minerals.

Graphite forms a minor constituent (5-10% of the mode) of pelitic schist, but in part accounts for its dark color in outcrop. Graphite is in contact with all other major minerals and is most commonly aligned with the phyllosilicates. In porphyroblasts, graphite is usually the dominant inclusion, displaying linear, curvilinear and folded patterns (Plate 3.5).

Plagioclase is the most abundant porphyroblast in pelitic units. Plagioclase accounts for 10-20% of the mode, is in contact with all other major minerals, and usually contains inclusions of graphite, white mica (phengite), and clinozoisite/epidote. On average the porphyroblasts are 1 – 1.5 mm in diameter, subhedral to anhedral in shape, partly corroded, and have first order gray through weakly yellow interference colors

(Plate 3.5). Based on the Michel-Levy optical technique plagioclase compositions range from albite to oligoclase. However, the technique could be applied to only a few grains

since most were untwinned or in unsuitable orientations for analysis. Hence, the full

range of plagioclase compositions is not known with certainty. Inclusions within

plagioclase porphyroblasts tend to be aligned at angles to the external foliation. 32

Plate 3.5 Photomicrograph of pelitic schist showing albite porphyroblast (upper center) preserving linear to curvilinear graphite inclusions (S1) at high angle to matrix foliation (S1/S2) is considered to have overgrown S1 prior to the development of S2. 33 Interbedded Pelitic and Psammitic Schist

Interbedded pelitic and psammitic schist is in depositional contact with other lithologies, and is distinct from pelitic units in that it is rhythmically layered on a centimeter scale. Psammitic layers are generally 3 to 10 cm thick, whereas the pelitic layers average only 1 to 2 cm. Individual psammitic layers may be more than 1 m thick

(Plate 3.6). Outcrops range from several meters to more than 20 m across. The psammitic layers are very fine grained, medium gray-white in color, and occasionally shiny on the dominant foliation surfaces where structurally aligned muscovite (phengite) is present. Lamination within the psammitic layers occurs on a 2 – 4 mm scale, separated by very thin sheets of mica. Quartz occupies more than 80% of the mode. The fine- grained pelitic layers are medium to dark gray in color, and somewhat shiny on the main foliation surfaces where chlorite and phengite are abundant. Subangular to subrounded white to gray quartz and feldspar grains (40% of some pelitic layers) average 0.2 – 0.4 mm in diameter. The feldspar grains are distributed rather homogeneously throughout the pelitic layers and are most likely porphyroblasts.

In thin section, the pelitic layers are like those described in the preceding section on pelitic schist. The psammitic layers however, are dominated by quartz, with lesser amounts of phengite, chlorite, and plagioclase. Quartz grains tend to be slightly larger than those within the pelitic layers, yet share the same characteristics. Both phengite and chlorite appear to be parallel to the main foliation and are in contact with all minerals

(Plate 3.7).

Garnet was observed to co-exist with biotite at one locality in the psammitic layers. The garnet porphyroblasts are mostly subhedral, account for 2% of the mode, and 34

Plate 3.6 Interbedded pelitic and psammitic schist showing cm-scale alternation of mica- and quartz-rich layers.

Plate 3.7 Photomicrograph of interbedded pelitic and psammitic schist showing layers of pelite (right) and psammite (left) defined by varying modal abundances of quartz and bedding-parallel phyllosilicates. 35 average 0.15 mm in diameter. Their rims are largely retrogressed to brown-colored chlorite, suggesting an iron-rich composition. The association of garnet with retrograde iron-rich chlorite and its black to dark red color suggests it is almandine. The garnets are pretectonic with respect to the composite external fabric, which wraps about the grains

(Plate 3.8).

Metachert

Interbedded microcrystalline siliceous and fine-grained siliceous-micaceous layers occur in proximity to the serpentinite mass and are pale gray-brown in color (Plate

3.9). The fine-grained layers are slightly lighter in color, with grain sizes averaging 0.2

– 0.5 mm, and layer thicknesses approximating 1 – 2 cm. The microcrystalline layers

(chert?) average 2 – 5 cm in thickness, and have variable coloration, with pale green- brown mottled with dark red-brown predominating. Quartz grains are stretched and flattened to dimensions averaging 0.1mm by 0.05 mm. Thin, milky-white quartz veins

(5% of outcrop) averaging 0.5 to 1 cm in thickness, crosscut the layering in various orientations.

The thicknesses of the siliceous and micaceous layers of this unit are very similar to those of the interlayered pelitic and psammitic schists of the field area. It is possible that the metachert, which is in close proximity to the tectonic fault contact with the serpentinite, is in fact a metasomatized unit of the interbedded pelitic psammitic schist.

Graphitic Schist

Schists rich in graphite are found in depositional contact with other pelitic lithologies and occasionally along fault contacts. Outcrop widths vary from a few meters up to 20 meters. The very fine-grained graphitic schist is dark gray in color and in some 36

Plate 3.8 Photomicrograph of retrogressed garnet porphyroblast within a psammitic layer of interbedded pelitic and psammitic schist. Matrix foliation wraps about garnet, suggesting pre- to syntectonic garnet growth relative to S1/S2.

Plate 3.9 Hand sample of metachert showing interbedded microcrystalline and siliceous- micaceous layers. 37 areas appears rusty due to the presence of weathered sulfides (Plate 3.10). The main foliation surface has a moderate sheen due to the structural alignment of graphite (and/or chlorite). Similar to the metapelites, the structural fabric is intensely developed in this lithology, however, probably as a result of its high graphite content.

In thin section, graphite and quartz occur in approximately equal volumes. The graphite occurs as elongate grains that parallel the main foliations. Quartz grains are weakly stretched and flattened parallel to the dominant foliation. They display undulose extinction, but are partly recrystallized indicating dynamic recrystallization or post deformational annealing (Plate 3.11).

Metavolcaniclastic Schist

Fine-grained volcaniclastic schist comprising rhythmically alternating light blue-

green and dark gray layers occurs in depositional to sharply fault bound contact (Plate

3.12) with other lithologies. The nature of its contacts with metasedimentary units is

invariably subvertical and approximately parallel to the main foliation (S1/S2). Light colored layers may be several centimeters to more than 30 meters thick, whereas the dark layers typically show thicknesses of a few centimeters (Plate 3.13). The fine-grained volcaniclastic units are relatively homogenous, with very little grain size variation and layering attributable only to that produced by the main foliation. An occasional lustrous sheen on the main foliation surface is due to the parallel alignment of chlorite, actinolite, or phengite. A prominent intersection lineation is produced by the intersection of a crenulation cleavage (S3) with the main foliation.

According to Ramírez-Ramírez (1992), the light colored layers consist of chlorite, actinolite and epidote, whereas the major minerals in the dark layers are chlorite, 38

Plate 3.10 Outcrop of graphitic schist showing intensely developed structural fabrics reflecting its high graphite content. The outcrop has a rusty appearance because of weathered sulfides.

Plate 3.11 Photomicrograph of graphitic schist showing a high modal abundance of graphite and quartz, the two predominant minerals that define this unit. 39

Plate 3.12 Outcrop of mafic volcaniclastic, the light blue-green color and fine-grained texture of which distinguish it from other units seen here in sinistral fault contact with pelitic schist. 40

Plate 3.13 Volcaniclastic schist with light blue-green and dark gray layers, the major minerals of which include chlorite, actinolite and epidote. Inferred recrystallized rock fragments and mineralogy indicate a mafic volcaniclastic protolith. 41 actinolite, albite, and quartz. The clastic nature is inferred in thin section from mosaic patches of epidote, albite, and actinolite, which may represent recrystallized rock fragments.

In thin section, 0.4-0.8 mm fibrous actinolite, phengite, and lesser amounts of chlorite define the main foliation, and each is in contact with all major minerals, occupying 60% of the mode. These minerals wrap around plagioclase and clinozoisite porphyroblasts, and partly penetrate their grain boundaries. Other minerals present include, in decreasing order, porphyroblastic plagioclase, clinozoisite, and calcite.

Chlorite is also found as a pressure shadow mineral for all porphyroblasts.

Subidioblastic to xenoblastic plagioclase averages 1 mm in length and accounts for 25% of the mode. Given that no lamellae are present, and only simple twinning is noted, the plagioclase composition is likely that of albite. Unlike the graphite-bearing plagioclase porphyroblasts in the metasediments, however, these are devoid of graphite.

Instead, they most commonly include microlites of mutually aligned phengite in their cores and have relatively inclusion-free rims, except for minor fabric-forming actinolite penetrating grain boundaries (Plate 3.14).

Clinozoisite is found as compositionally zoned porphyroblasts with cores of epidote or Fe3+-rich clinozoisite (upper first to lower second order birefringence) and rims of more aluminous clinozoisite, displaying anomalous blue to yellow interference colors. However, some of the porphyroblasts are unzoned and consist solely of clinozoisite. In contact with all other major minerals, elongate clinozoisite occurs as anhedral to subhedral grains that range in length from 0.4 mm to 0.8 mm, and accounts for 10-15% of the mode. Clinozoisite long axes generally lie within the main foliation. 42

Plate 3.14 Photomicrograph of a plagioclase (Ab) porphyroblast in mafic volcaniclastic at extinction. Core displays mutually aligned phengite and actinolite (At) inclusions at angles to the external fabric, whereas the rim consists of actinolite that parallels the external foliation, suggesting multistage growth. Calcite (Cc) rims plagioclase. Clinozoisite (Ec) is also within the fabric. 43 However, instances exist where the long axes of clinozoisite are at high angles to the foliation, such that large pressure shadow zones have developed and are occupied by chlorite (Plate 3.15).

Calcite, which is a very minor component to the rock (1-2% modal abundance), occurs as anhedral grains averaging 0.2-0.4 mm in length. An association with plagioclase and clinozoisite is prevalent, and rarely is calcite found solely within the actinolite/chlorite foliation (Plate 3.14). 44

Plate 3.15 Photomicrograph of clinozoisite (Ec) porphyroblast at high angle to the foliation. Fibrolitic actinolite wraps about the grain, indicating clinozoisite growth was pre- to syn-kinematic with respect to the external foliation. Pressure shadows are occupied by chlorite (Ch). 45

Metaigneous Units Metabasalt

Metabasalts appear to be in depositional contact with other lithologies, as well as

in occasional fault contact parallel to the main foliation. The aphanitic-textured

metabasalts are much less foliated than other schist lithologies, presumably due to their

smaller phyllosilicate content (Plate 3.16). Their color is a consistent medium gray-

green, occasionally weathered to a rusty-brown due to oxidation of Fe2+-bearing minerals. Metabasalt outcrops typically do not exceed a few meters in extent.

In thin section, varying amounts of chlorite, plagioclase, quartz, epidote, calcite, and iron oxides account for the majority of minerals present (Plate 3.17). Chlorite occurs as a very fine fibrous matrix mineral that is pleochroic green under plane polarized light, and is responsible for the greenish gray hue of the outcrops in which it is found. Under crossed polars, chlorite is generally bluish in color, which may indicate a substantial fraction of iron within its structure. In contact with all other major minerals, chlorite accounts for 40% of the mode. Late veins that crosscut the main foliation are chlorite rich, and also contain trace amounts of calcite and pyrite.

Plagioclase porphyroblasts are usually untwinned, or occasionally display simple twinning, suggestive of an albite composition. Their cores are rich in muscovite that appears to be in random orientations. The grain shapes are largely anhedral and average

0.5-1.0 mm in length. In contact with all other major minerals, plagioclase modal abundance is estimated at 25%.

46

Plate 3.16 Hand specimen of metabasalt showing more massive (less foliated) appearance relative to other schist lithologies.

Plate 3.17 Photomicrograph of metabasalt showing a weak foliation. Component minerals include chlorite (Ch), plagioclase (P), quartz (Q), epidote (E) calcite, and iron oxides or sulfides. 47 Quartz occurs as elongate strained grains that lie within the main foliation. The length to width ratio of quartz minerals is generally 4:1, and lengths average 1-2 mm.

Quartz is found throughout the rock and is in contact with all other major minerals. Its abundance represents 20% of the mode.

Epidote occurs as very small subhedral grains often elongate along their c-axes, and are found predominantly within the chlorite fabric. Interference colors reach second- order red and blue. Grain lengths normally do not exceed 0.1-0.2 mm. The epidote grains represent 5% of the mode.

Calcite is a minor component in the metabasalts, accounting for 2-3% of the mode, and it is in contact with all the major minerals. However, it is most often associated with opaque minerals (iron oxides), where it partly surrounds these grains.

Serpentinite and Metagabbro

Serpentinite

The serpentinite body, approximately measuring 0.5 x 10 km, is in tectonic contact with the metasediments of the Granjeno Schist. Outcrops of serpentinite in the southwest block of the anticlinorium extend over large areas as the result of quarrying activity.

Serpentinite-metasediment contacts are predominantly subvertical but also include moderately dipping surfaces (Plate 3.18). The strongly penetrative foliations, heterogeneous mineral lineations, and fissile nature of the serpentinite, (a response to brittle deformational events) proved too complex to decipher over much of the outcrops observed. In rare instances, however, shear zones were located within the serpentinite 48

Plate 3.18 Pale-green serpentinite in tectonic contact with the Granjeno Schist. The juxtaposition of these units, here at moderate angles, is usually subvertical. 49 mass that had coherent structure. The intensely foliated serpentinite varies in color from dark green-black and blue to pale yellow-green and gray, usually with a shiny luster due to the presence of structurally aligned lizardite and chrysotile serpentine (Ramírez-

Ramírez, 1992). Igneous textures were not observed in outcrop, but loose samples collected within the quarry, and are considered to be locally derived from the mining operations, show relict cumulate texture. These samples are massive, gray-green to green-black, with pale green subrounded pseudomorphed phenocrysts (serpentine mineral replacing olivine or Mg-pyroxene) averaging 0.5 cm in diameter. The groundmass is very fine grained and varies in color from dark green to reddish-brown.

Metagabbro

Mafic intrusives represented by metagabbro occur in association with the

serpentinite body. Contacts with the serpentinite were not directly observed, but the

metagabbro is in sinistral strike-slip contact shown by the metasediment fabric rotation

into parallelism with the fault. The metagabbro was observed mostly as float along and

to either side of an abandoned road used for access to an asbestos quarry within the

serpentinite body. Float of the metagabbro consists of cobble to boulder-sized fragments.

The fragments are typically weathered and subangular to subrounded in shape,

presumably due to the metagabbro’s non-foliated, homogenous texture (Plate 3.19).

External weathered surfaces are mottled white, gray and pale brown, and display a

coarse-grained hypidiomorphic-granular texture in which pale gray to white plagioclase

averaging 1 cm in length account for approximately 75% of the rock. Amphibole

(retrogressed hornblende) occupies approximately 25% of the mode and occurs as dark

brown minerals averaging 1-2 cm in length. 50

Plate 3.19 Metagabbro boulder with hypidiomorphic-granular texture consisting largely of retrogressed plagioclase and hornblende. 51 Where the metagabbro occurs in sinistral contact with metasediments, the original cumulate texture is preserved, but the mineralogy largely consists of metamorphic chlorite, actinolite, clinozoisite, and calcite. The original phenocrysts of plagioclase are highly altered to muscovite and are associated with calcite. Fibrous masses of actinolite and brown chlorite occupy more than 50% of the mode. Actinolite is in places overgrown by recrystallized plagioclase. The porphyroblasts of clinozoisite show anomalous blue interference colors and partly overgrow actinolite and are mildly altered to calcite.

52 IV. Structural Geology

Introduction

The rocks of the Granjeno Schist have undergone extensive deformation,

recording a minimum of four penetrative phases in the outcrops observed in the field

area. Based on structural style or overprinting relationships, these four deformational

phases are designated D1 – D4. Figures 4.1 through 4.11 show equal area stereographic

projections of the various phases of deformation found in the field area, as well as their

locations on the field map. The structural data have been subdivided into four regions

(Figure 4.12) within the map area, based primarily on the location and access to the

various outcrops. These structural subareas lie (1) southwest of a major sinistral fault in

the south tributary to the Cañon del Novillo, (2) northeast of this sinistral fault, (3) on the

road sections accessing the serpentinite quarry, and (4) on the north tributary to the

canyon. The main river into which the tributaries flow is not within the map area, but

exposes outcrops of the Novillo Gneiss. Details of the structure are described in this

chapter.

D1 Structures

The first recognized phase of deformation in the Granjeno Schist (D1) consists of a bedding-parallel foliation (S1) that is axial planar to mesoscopic isoclinal folds (F1).

Bedding-parallel quartz veins folded by F1 provide the best record of this phase of

deformation, but microstructural investigation shows that these folds are also preserved in

plagioclase porphyroblasts as well as within the composite matrix foliation of the pelitic

schists. F1 fold axes plunge southeast from 0° to 78° degrees, with moderate to steep

53

Figure 4.1 D1/D2 in Subarea 1. Crosses are F1/F2 fold axes, circles are poles to S1/S2.

Figure 4.2 D3 in Subarea 1. Crosses are F3 fold axes, circles are poles to S3.

54

Figure 4.3 D4 type locality in Subarea 1. Open cross is F4. Open square is pole to F4 axial plane. Solid crosses are L3. Solid squares are poles to S3. Circles are poles to S1/S2. L3 has a clockwise rotated sense of 30°-40° relative to F4, shown by the 35° small circle about F4. 55 (a)

(b)

Figure 4.4 Structural relationship in Subarea 1 (a) on limbs of F1/F2 folds, and (b) at F1/F2 hinges. Crosses represent fold axes, and circles represent poles to foliation. Solid symbols are D1/D2. Open symbols are D3. 56

Figure 4.5 D1/D2 in Subarea 2. Crosses are fold axes, circles are poles to foliation. Solid symbols are D1. Open symbols are D2.

Figure 4.6 D3 in Subarea 2. Crosses are F3 fold axes, circles are poles to S3. 57

Figure 4.7 Structural relations in Subarea 3. Crosses are fold axes, circles are poles to foliation. Solid symbols are D1/D2. Open symbols are D3.

Figure 4.8 Structural relations in Subarea 4. Crosses are fold axes. Circles are poles to foliation. Solid symbols are D1/D2. Open symbols are D3.

58

Figure 4.9 Major dextral faults. Blue squares represent poles to bounding faults (i.e. Granjeno Schist/leucogranite contacts). Black circles represent main dextral faults within Granjeno Schist. Triangles represent slickenside lineations showing movement of leucogranite/Novillo Gneiss towards the southeast relative to the schist. Dashed great circle represents average orientation of dextral faults within the Granjeno Schist. 59

Figure 4.10 Major sinistral faults. Squares represent poles to fault planes. Triangles represent slickenside lineations showing movement of northeastern sides of faults towards the northwest. Dashed great circle represents average sinistral fault orientation.

Figure 4.11 Average sinistral and dextral fault orientations (great circles) within the Granjeno Schist, and F4 fold axes (crosses). The folds are associated with the faults and are compatible with the overall dextral strike-slip regime. 60

Figure 4.12 Geology map of the field area showing structural subareas based primarily on the location and access to the various outcrops. See Appendix map for geological details. 61 angles predominating (Figures 4.1 – 4.8). In thin section, the foliation is thought to be a composite (S1/S2) fabric defined by the parallel alignment of phengite, chlorite, and graphite. Graphite inclusions within the plagioclase porphyroblasts of metasedimentary units, and the aligned microlites of white mica in the plagioclase porphyroblasts of the metaigneous lithologies, show linear and folded patterns that are oriented at angles to the external foliation. These fabrics are interpreted as relicts of the first foliation in the schist

(S1), which were preserved as the porphyroblasts grew post-kinematically with respect to

the D1 deformation (Plate 4.1; Plate 3.14). The composite foliation in metasedimentary

units similarly preserves rootless isoclinal closures between foliation planes that are

defined by graphite and phengite, and have limbs that are parallel to the composite

foliation, implying that S2 is a crenulation cleavage of S1 (Plate 4.2; see also Plate 3.3).

These are interpreted as F1 folds. Lineations associated with D1 have not been found,

suggesting that either simple shear was not a major component of D1, or that subsequent deformation and metamorphism has entirely overprinted these early structures. However, the angularity between the preserved microfabric in porphyroblasts and the external fabric indicates that the porphyroblasts were rotated during D2, suggesting a component of simple shear.

D2 Structures

The second recognizable phase of deformation in the Granjeno Schist (D2) produced a bedding-parallel foliation (S2) that is axial planar to mesoscopic isoclinal

folds (F2), and crenulated the S1 fabric. F2 folds are most evident where S2 is at high angles to the bedding-parallel S1 foliation in F2 closures, particularly where F1 closures occur near F2 hinges. On the limbs of F2 folds, S2 becomes mutually parallel and, hence, 62

Plate 4.1 Photomicrograph of an albite (Ab) porphyroblast in pelitic schist. Graphite (G) inclusions define fold patterns considered to be relicts of the first foliation that were preserved as the porphyroblast grew post-kinematically with respect to D1. Other minerals include quartz (Q), chlorite (Ch), and phengite (M).

Plate 4.2 Photomicrograph of rootless isoclinal folds defined by graphite within the composite foliation of pelitic schist. These folds are similar to those preserved within the albite porphyroblasts, suggesting they are products of the same deformation. 63 indistinguishable from S1. Therefore, S1 and S2 form a composite foliation (S1/S2) throughout much of the Granjeno Schist (Plate 4.3) and, as such, S2 is defined by the

same phyllosilicate mineralogy (phengite and chlorite) as S1. Measurable F2 fold axes are

rare, but plunge southeast at moderate (60° and 67°) angles (Figure 4.5). In locations

where F2 folds were recognized, F1 fold axes lie at about 30° to F2 in the S1/S2 plane, producing shallowly SE-plunging F1 axes on the west limbs of F2 folds, and steeply SE-

to NE-plunging F1 axes on their east limbs. Given that the orientations of the F1 and F2 fold axes are similar, and their styles of deformation as seen in outcrop are difficult to distinguish unless F1 hinges are found near F2 closures, separation of the two phases is largely incomplete. Stereonets showing D1-D2 fold data (Figures 4.1-4.8) consequently show only two definitive F2 axes, although others are likely present among the plotted axes. The mineralogy of the S2 foliation in F2 closures was not observed in thin section, except in one sample of graphitic schist (Plate 4.4; Plate 4.5). But, since the composite foliation in this sample is defined by graphite alone, any distinction in mineralogy between S1 and S2 was precluded. This issue is further discussed in the following chapter

in sections describing the metamorphic conditions during D1 and D2. In thin section, evidence of D2 is preserved, in part, in the rims of the plagioclase porphyroblasts of meta- igneous units, where the external foliation penetrates porphyroblast grain boundaries

(Plate 3.14). However, the external fabric wraps around these porphyroblasts, indicating that growth of the rims must have been completed by early D2. Lineations associated with D2 have not been found, indicating, like D1, that either simple shear was not important during D2, or that subsequent deformation and metamorphism have entirely overprinted these structures. 64

Plate 4.3 F2 isoclinal fold, defined by the folding of bedding and the S1 bedding-parallel foliation.

Plate 4.4 Outcrop of graphitic schist showing isoclinal folds defined by quartz veins. Inset shows that F1 closures are found within this isoclinal F2 hinge zone. 65

Plate 4.5 Photomicrograph of the F2 closure shown in the Plate 4.4. The S1 and S2 foliations, here at high angles to one another, are both defined by graphite, which hinders their distinction. 66 D3 Structures

The third phase of deformation (D3) produced a prominent crenulation cleavage (S3) that is axial planar to mesoscopic and microscopic folds (F3). These structures refold earlier

structures and are particularly prominent in rocks containing large proportions of modal

micas (Plate 4.6). In thin section, the crenulation cleavage is typically defined by the

alignment of phyllosilicates (phengite and chlorite), either as a result of folding (zonal

crenulation cleavage), or through the development of discontinuities along the cleavage

planes (discrete crenulation cleavage). Associated with S3 is a crenulation lineation (L3) on the S1/S2 foliation surfaces that is far more widely developed than F3 folds of measurable size. S3 strikes NE-SW with predominantly moderate to steep angles of dip.

F3/L3 axes generally plunge SSW to SSE at shallow to steep angles (5° to 84°), with moderate angles predominating (Figures 4.2-4.8). The estimated amount of shortening associated with the crenulation of earlier fabrics appears to be dependent on lithology.

Using the method described by Davis and Reynolds (1996), estimates of shortening in thin section range from 30 - 50% in pelitic lithologies (averaging 30%) and 10-30% in psammitic or mafic lithologies (averaging closer to 10%) (Figure 4.13; Plate 4.7; Plate

4.8).

In the southwest corner of the field area (Subarea 1), near the tectonic contact between the Granjeno Schist and the serpentinite body, F3 axes plunge at predominantly low angles to the south, and shallow-dipping S3 strikes NNW to NNE on limbs of macroscopic F1/F2 tight to isoclinal folds. However, in the hinge zones of F1/F2 structures, S3 is notably steeper, likely as a result of D4 structures, which are addressed in

the following section. In no other areas of the Granjeno Schist is S3 shallow dipping, 67

Plate 4.6 Small-scale F3 folds in metapelite defined by the refolding of S1/S2 to produce a crenulation cleavage (S3).

Figure 4.13 Diagram used to estimate the amount of shortening associated with the D3 crenulation of the S1/S2 fabric (Davis and Reynolds, 1996). 68

Plate 4.7 F3 hinge zone in metapelite with estimated 30-50% shortening, based on method of Davis and Reynolds (1996).

Plate 4.8 S3 in mafic metavolcaniclastic with 10-30% shortening, based on method of Davis and Reynolds (1996). 69 which may indicate that the effect of D4 structures on preexisting ones is more significant outside Subarea 1 and/or that the development of D3 structures is heterogeneous over the field area.

D4 Structures

The fourth phase of deformation (D4) produced steeply inclined, moderately SE-

plunging close folds (F4) that do not have an associated cleavage. These structures fold the S3 crenulation cleavage and the crenulation lineation (L3; Plate 4.9). The position, size, and wavelength of F4 folds are notably variable and to some degree can be correlated with rock type, structural orientation, and relationship to faults. Mesoscopic folds are more common in the metapelites, where ductile deformation is favored. In the extreme southwest portion of the area, close to the serpentinite quarry (Subarea 1), a correlation exists between well-developed mesoscopic F4 folds and the hinges of macroscopic F1/F2 folds (Appendix and Figure 4.4). The dominant strike throughout the

area, defined by the S1/S2 fabric and bedding, is north to NNW, which correlates with limbs of the F1/F2 isoclinal folds. This orientation is also subparallel to the F4 flattening plane, implying that folding of layers with this orientation prior to the onset of D4, would be limited. However, in the hinges of the large isoclinal structures, where bedding/foliation trends are other than north to NNW, the layering may have been more susceptible to F4 fold development. It is in these areas that mesoscopic F4 folds are most

readily recognized. The following section provides a detailed description and analysis of

the F4 type locality in the southwest corner of the field area. 70

Plate 4.9 Mesoscopic F4 folds defined by the S1/S2 composite foliation. D3 structures are folded by D4, but are oblique to F4 axes.

71 Structural Analysis

In order to determine the amount of shortening produced by an individual F4 fold, the distance between inflection points measured along the folded layers in the profile plane was compared to the straight line distance between inflection points. This method yields an estimate of 20% shortening, which is less than the 30% shortening required for a suitable rock to develop a slaty cleavage (Park, 1989). On the NE limb of the F4 fold,

L3 plunges gently NNW, whereas on the SW limb, it plunges moderately to the SSW.

Plotting the orientation of L3 around the F4 fold shows that L3 maintains a nearly constant

angle (between 30° and 40°) with respect to the F4 fold axis, producing a small circle on a

stereoplot. Such deformed lineation patterns are characteristic of flexural shear folds

(Ramsey, 1967). Similarly, the S3 crenulation cleavage, which is oblique to the S1/S2 layers that define the fold, is deformed into a cone shape such that it maintains an angle of approximately 80° to F4 as it is bent about the F4 fold axis. However, since the F4 profile is not that of a parallel fold (see below), mechanisms other than ideal flexural shear were presumably involved in its development.

Detailed geometric classification of the fold was based on dip isogon patterns and changes in the thickness of the folded layers. The dip isogon method (Plate 4.10; Figure

4.14) shows that the west limb spans geometric classifications between weakly class 3

(divergent isogons), through class 2 (parallel isogons = similar folds), to weakly class 1C

(convergent isogons) folds. The east limb similarly spans class 2 to weakly class 1C folds. Plotting changes in layer thickness produces the same results (Figure 4.15).

72

Plate 4.10 Profile of F4 fold used for its geometric classification. 73

Figure 4.14 The dip isogons for F4 fold are weakly divergent (class 3), to parallel (similar) to weakly convergent (class 1C). Lines defining fold trace the foliation and do not indicate compositional layering. 74 (a)

(b)

Figure 4.15 Plots showing changes in layer thickness across the F4 fold. The west limb (a) spans similar to weakly class 1C folds. The east limb (b) spans similar to weakly class 3 folds. S3 is subparallel to S1/S2 on the east limb, producing a thin limb, whereas it is at high angles to S1/S2 in the west limb, which may account for the slight difference in folding style. α is the limb inclination relative to the datum. Relative thickness is the ratio of true thickness (tα) measured at a location on a fold limb to thickness (to) measured in the fold’s hinge (Davis and Reynolds, 1996). 75 The slight difference in folding style between limbs may be related to the fact that S3 parallels S1 and S2 in the east limb of the fold, producing a relatively thinned limb, whereas S3 is at a high angle to S1 and S2 in the west limb.

A progressive change in the style of folding from layer 1 to 4 (Figure 4.14) is to

be expected, since in outcrop, the fold geometry when viewed in profile does not remain

constant. Given that Class 2 similar folds generally develop when rocks are in a very

ductile state, typically during medium to high grade metamorphism (Park, 1989), it is

unlikely that the near similar F4 fold geometry developed by oblique shear. Instead,

buckling of the rocks by layer-parallel shortening followed by homogeneous flattening

may account for the fold style. If the folded pelitic unit was not adjacent to an actively

buckling competent layer, then it is likely to have deformed less by flexural shear and

more by homogeneous flattening. This may account for the near similar folding style and

for the fact that L3 does not maintain a perfectly constant angle with respect to the F4 fold axis.

Relationship to faulting

Faults are prevalent in the map area, yet some generalizations can be inferred.

Dextral and sinistral fault zones with foliated fault gouge range from a few centimeters up to a meter across, are steeply inclined, and have average strikes to the north and

WNW, respectively. Given the similarity in style of faulting and their orientations, as well as their apparent association with fault-bound F4 folds (Figures 4.9-4.11), a

relationship between D4 and an overall dextral strike-slip regime is inferred (Plates 4.11-

4.13). 76

Plate 4.11 Ductile shear zone separating the Granjeno Schist from the leucogranite showing dextral S-C fabric. The S-fabric is linked with D2 since the shear post-dates the intrusion, and is overprinted by D3 structures (see Plate 6.1).

Plate 4.12 Major sinistral fault in south tributary separating dark Granjeno Schist from metagabbro. The orientation of sinistral faults is geometrically linked to the dextral strike slip regime. 77

Plate 4.13 Ductile shear zone in serpentinite showing dextral S-C fabric. The orientation and style of deformation in this shear zone is nearly identical to that of the dextral shear separating the Granjeno Schist from the leucogranite, and therefore is also associated with D2. 78 V. Geochronology

Samples collected for geochronological analysis included the leucogranite and a quartz keratophyre unit within the Granjeno Schist on the road to the serpentinite quarry.

Zircons separated from the leucogranite were analyzed by Ken Cameron at the University of California, Santa Cruz, using the methodology described by Lopez et al. (2001). The analyses were conducted on single, irregular-prismatic, abraded zircons and small populations of prismatic grains. All the data are discordant and fall on a chord that intersects concordia at 990±46 Ma and 351±54 Ma (Figure 5.1; Table 5.1), which are interpreted to date the inheritance and the time of intrusion, respectively. The inheritance age is within the range of those reported from the Novillo Gneiss (Cameron et al., 2004).

Muscovite separates from the leucogranite and a quartz keratophyre unit within the Granjeno Schist were analyzed by 40Ar/39Ar laser step-heating thermochronology by

Annabel Ortega at Queens University, Ontario, using the methodology reported in

Keppie et al. (2004). Muscovite that defines the weak foliation in the leucogranite

yielded an Ar-loss profile with an age of 138 ± 10 Ma in the lowest temperature step rising to a plateau of 313 ± 7 Ma in the four highest temperature steps (Figure 5.2 (a);

Table 5.2). The plateau age is inferred to date cooling through ca 350oC (Purdy and

Jäger, 1976). The muscovite separate from the quartz keratophyre yielded concordant data with a plateau age of 300 ± 4 Ma (Fig. 5.2 (b); Table 5.2), and falls on the

Carboniferous-Permian boundary (Okulitch, 2003).

79

N28 Paleozoic Granite

0.15 900 E3 E2 800 0.13 U

238 700 A E1 Pb/ 0.11 D2

206 600 D1 Intercepts at C 351 ± 54 & 990 ± 46 Ma 0.09 F2 G 500 MSWD = 66 0.07 0.5 0.7 0.9 1.1 1.3 1.5 207Pb/235U

Figure 5.1 U-Pb concordia diagram for zircons from the leucogranite. The lower intercept is considered the intrusive age, whereas the upper intercept is considered an inheritance age, and is concordant with that of the Novillo Gneiss. 80

Table 5.1 U-Pb data for zircon in leucogranite. * Radiogenic Pb. abr=abraded, prsm=prismatic, irreg=irregular, 1:2=aspect ratio, com=common, F=fraction, fil=filament, Tot=total.

81

(a) (b)

Figure 5.2 40Ar/39Ar apparent age spectra of (a) muscovite in leucogranite and (b) phengite in quartz.

82 (a)

(b)

Table 5.2 40Ar/39Ar data for (a) muscovite in leucogranite and (b) phengite in quartz keratophyre. (a) Mass: 1.0 mg. Volume 39K: 4.71 x 1E-10 cm3 NTP Integrated Age: 280.27±5.60 Ma. Initial 40/36: Linear regression has positive slope. Correlation Age: 288.66±50.26 Ma (98.3% of 39Ar, steps marked by >). Plateau Age: 313.46±7.34 Ma (65.0% of 39Ar, steps marked by <). J = .007591± 0.000068. (b) Mass: 1.0 mg. Volume 39K: 6.49 x 1E-10 cm3 NTP. Integrated Age: 294.11±3.65 Ma. Initial 40/36: 1046.06±3489.73 (MSWD = 2.44, isochron between 0.29 and 2.41). Correlation Age: 288.66±50.26 Ma (98.3% of 39Ar, steps marked by >). Plateau Age: 299.68±3.77 Ma (85.4% of 39Ar, steps marked by <). J = 0.007671± 0.000052. 83 VI. Deformation and Metamorphism

Introduction

In this chapter, the relationships between metamorphism and deformation are

discussed. This, in conjunction with new age data, is then used to constrain the nature

and timing of the various tectonothermal events (Figure 6.1) that affect the Granjeno

Schist and the serpentinite-metagabbro body, and their potential tectonic significance.

D1 Tectonothermal Event

The first phases of deformation (D1) and metamorphism (M1) are the only ones known to predate intrusion of the leucogranite, which contains xenoliths of the Granjeno

Schist, since, as will be discussed in the following section, D2 records dextral shear between the Granjeno Schist following the leucogranite intrusion. Therefore, D1 must be older than the 354±54 Ma lower intercept age that is taken to date the leucogranite’s emplacement. Given the protolith lithologies of the Granjeno Schist (pelitic sediments, mafic volcanics) and their association with serpentinite and gabbro, a marine depositional environment, perhaps within an accretionary prism or forearc basin setting, seems likely.

D1 may therefore preserve an early phase of ocean closure and ophiolite obduction.

Since the grade of metamorphism inferred from the rare preservation of biotite-garnet

falls in the mid- to high-greenschist facies (garnet zone), it is unlikely that D1 records a subduction event, at least, not to any great depths, such as those where blueschists evolve.

The emplacement of the serpentinite and metagabbro is also likely to be a manifestation of D1 deformation, given that multidirectional veins produced by expansion during

serpentinization are crosscut by D2 shear zones. 84

Figure 6.1 Temperature-time diagram for the Granjeno Schist showing mineral parageneses in relation to deformational phases and geochronological data. D1 is separated in time from D2 by the intrusion of the leucogranite. D2-D4 may be parts of a progressive deformational event associated with dextral shear and spanning the Late Carboniferous and Early Permian. Width of horizontal black bars indicates the length of time over which individual minerals grew relative to phases of deformation. The width of vertical gray columns indicates the duration of deformation relative to metamorphic mineral growth. 85 The D1 deformation in metapelitic lithologies produced a bedding-parallel

foliation defined by the alignment of biotite and muscovite. The widespread absence of

preserved garnet suggests that, in general, the metamorphism (M1) accompanying this deformation did not exceed the biotite zone of the greenschist facies. However, a single occurrence of garnet porphyroblasts indicates that metamorphism locally reached the garnet zone. Some of the garnet porphyroblasts show partial rotation during later stages of growth, indicating that M1 was syn- to post-tectonic with respect to D1. The preservation of S1 fabric-forming actinolite in the albite porphyroblasts of meta-igneous lithologies is also consistent with moderate greenschist facies metamorphism. The metagabbro in sinistral fault contact with metapelite in the south tributary, has a mineralogy that includes actinolite, albite, clinozoisite, calcite, and chlorite, which, is typical of greenschist facies metamorphism. However, it is uncertain whether the present assemblage preserves M1, or is only that of M2.

D2 Tectonothermal Event

The second phase of deformation (D2) recorded in the Granjeno Schist is known to postdate the intrusion of the leucogranite which separates the Granjeno Schist from the

Novillo Gneiss within the field area, and is associated with movement along the faults that separate these units. Evidence for this is derived from the fact that the leucogranite encloses xenoliths of the Granjeno Schist, and that ductile deformation in the shear zone separating the Granjeno Schist from the leucogranite is overprinted by D3 structures

(Plate 6.1). Geological investigation of this boundary reveals a D2 S-C fabric that is 86

Plate 6.1 S2-C fabric within dextral shear zone separating the Granjeno Schist from the 354±54 Ma leucogranite, here overprinted by S3. 87 indicative of dextral strike-slip kinematics. Given that the narrow shear zone is a manifestation of D2, estimates of the amount of strain and flattening within this zone can

be accomplished by analysis of the S-C fabric.

According to Davis and Reynolds (1996), the amount of strain (γ) due to simple

shear may be estimated by measuring the angle θ between the C and S planes and using

the equation γ = 2/(tan (2θ)), such that as θ incrementally decreases linearly, each γ

increment becomes progressively larger. The angle between S and C is approximately

40°, giving a value for γ of 0.35. The amount of flattening may be estimated by

measuring the ratio of the angle between the planes to that of the 45° angle that is

developed between them at the onset of simple shear. Using the S-C angle of 40°,

flattening in the shear zone is estimated to be 11%.

Estimates of percent lengthening and shortening in other areas of the Granjeno

Schist may be inferred from “clasts” in the layered volcaniclastic rocks that resemble

strain ellipsoids (Plate 6.2). Although variable in size, the clasts appear to share similar

ratios in their dimensions. Furthermore, veins that crosscut many of the clasts and occur

only within them are mutually parallel, suggesting that they were subjected, and

responded equally, to the ambient stresses. Therefore, by making the assumption that the

clasts were initially spherical, and knowing that they have been subjected to two phases

of intense deformation, a local estimate of net lengthening and shortening related to D1 and D2 may be calculated. The results, however, must be regarded as very approximate

because (1) the shape of the clasts prior to deformation is unknown, (2) F1 and F2 axes are at slight angles to one another, (3) the cross-sectional area of the clasts must be assumed 88

Plate 6.2 “Clast” resembling strain ellipsoid used to approximate net lengthening (82%) and shortening (45%) during D1 and D2. 89 to have remained constant during deformation, and (4) changes in clast dimensions as a result of post D2 deformation have not been taken into account.

If the width of the line in the plane occupied by the quartz vein (3 units) is taken to be that of greatest shortening, and the maximum length of the clast (10 units) is taken to be representative of the maximum stretching direction, an original radius of 5.5 units is obtained by setting the areas of an ellipse and circle equal to one another. Therefore, extension parallel to the long direction is given by (10-5.5)/5.5 = 0.82 (lengthening of

82%), and extension parallel to the short axis is (3-5.5)/5.5 = -0.45 (shortening of 45%).

The style of deformation (isoclinal folding and penetrative axial planar fabric) related to D2 is nearly identical to that of D1, yet the two episodes are separated in time

by the intrusion of the leucogranite. Hence, the structures associated with D2, must be related to some later stage in the accretionary process. Given that the leucogranite is likely Carboniferous in age, that it intrudes both the Granjeno Schist and Novillo Gneiss, and that unmetamorphosed rocks of similar age lying some distance above the Novillo

Gneiss contain Laurentian fossils (Stewart et al., 1999), D2 is thought to record dextral shear during the amalgamation of southern Laurentia and Gondwana.

The D2 deformation, like that of D1, produced isoclinal folds with a strongly

penetrative axial planar fabric. However, the minerals that define the S1/S2 foliation in the metasediments include muscovite (phengite) and chlorite (after biotite) without biotite. This suggests that the metamorphism (M2) that accompanied D2 did not exceed

the chlorite zone of the greenschist facies. Inclusion-bearing plagioclase porphyroblasts

in both metasedimentary and meta-igneous lithologies are considered post-kinematic with

respect to D1. Yet, they occasionally have relatively inclusion-free rims that are 90 penetrated by S2-defining minerals that remain parallel to the external S1/S2 fabric. This

suggests that rim growth on preexisting porphyroblasts is post-M1 and therefore a product

of M2 metamorphism. The external fabric is further observed to wrap around the albite porphyroblasts, suggesting that the onset of M2 was pre- to early-syntectonic with respect to D2.

The secondary growth that produced the occasional inclusion-free rims on the albite porphyroblasts may be an indication of depressurization, such that the stability field of albite is increased, allowing for continued metamorphic growth presumably at higher crustal levels near isothermal conditions (Jamieson and O’Beirne-Ryan, 1991;

White et al., 2001). If the temperature is raised above a critical level, albite growth ceases. Petrographically, this may account for the early syn-kinematic rim growth of albite, followed by the continuing development of the S1/S2 fabric. In this scenario, following the intrusion of the leucogranite, rapid exhumation in the initial stages of D2 may have occurred under nearly isothermal conditions. The continuation of D2 deformation further develops the S2 fabric, while growth of the porphyroblast rims ceases under the increased temperatures and pressures associated with D2. The combination of dextral shear and uplift (or unroofing) may indicate a transpressional regime.

Clinozoisite in the meta-igneous lithologies also show multistage growth. Iron-rich clinozoisite porphyroblasts tend to overgrow the S1 fabric, and, like albite, occasionally

have inclusion-free rims, indicating post-kinematic growth relative to D1. Iron-poor clinozoisite within the fabric, and the iron-poor rims on iron-rich clinozoisite cores, appear to be an early syn-S2 fabric development, since their inclusions are subparallel to the composite external fabric, and are partly wrapped by the S1/S2 fabric. In summary, 91 M2 is evidenced by early syn-kinematic rim growth on albite and iron-rich clinozoisite

porphyroblasts, and with the iron-poor clinozoisite porphyroblasts within the fabric.

D3 Tectonothermal Event

The orientation of D3 structures is highly variable for reasons that appear to be

related to the fact that the S3 crenulation cleavage is oblique to the S1/S2 composite foliation, which defines the geometry of subsequent F4 folds. This has the effect of folding S3 into cone shapes. Unfolding of S3 about F4 structures, where possible,

produces ambiguous results, but F3 axes plunge at shallow angles to the south and SW or north and NE. In the southwest corner of the field area, where they were isolated from

D4 deformation, and may therefore preserve their original orientations, F3/L3 axes plunge shallowly south and S3 is shallow dipping.

The D3 deformation produced a crenulation cleavage defined by microfolding of

the S1/S2 fabric. Along planes defining the cleavage, both disjunctive and zonal cleavages are developed, even within a single sample, which may indicate a change from schistose/phyllitic plastic deformation to one more typical of slates. If this is the case, then, D3 may have been active through decreasing grades of metamorphism (i.e., from the chlorite zone to subgreenschist facies conditions) during continued exhumation or waning tectonothermal activity.

The pelitic schist in proximity to the serpentinite-metagabbro body has intensely developed D3 structures, such that some quartz veins folded by F3 have weak grain elongation and undulose extinction parallel to the S3 fabric. In the same F3 hinge zones, quartz within the mica-rich fabric remains weakly elongate and strained parallel to the main foliation, while other quartz-rich sections parallel to the fabric have predominantly 92 annealed textures. Such textures may reflect the varying ability of quartz to recrystallize, depending on whether other minerals obstruct its regrowth (i.e., the micas).

D4 Tectonothermal Event

Fold structures associated with D4 have been observed at various localities in the

field area, and are thought to be associated with the dextral strike-slip regime that led to

the final amalgamation of the Granjeno Schist, the Novillo Gneiss, and the serpentinite-

gabbro body. In the type section for F4, which is located in the macroscopic F1/F2 synformal hinge of Subarea 1, S0 in the hinge of F1/F2 is at a high angle to the main

dextral strike-slip fault planes, such that the F4 fold lies in the plane of extension with respect to the dextral strike-slip regime. Buckling of competent layers produced by the compressional forces of dextral strike-slip may therefore have been at a maximum in this area. Other areas of the Granjeno Schist, particularly at higher elevations where road cuts are plentiful, provide further evidence for post D1 through D3 folding. Here, the composite foliation dips at shallow to steep angles, but is ubiquitously rotated parallel to subvertical NW-SE faults where the foliation reaches steep angles of dip. This further strengthens the notion that the development of D4 structures is related to late movement

on the dextral strike-slip faults, and may indicate a compressional component

(transpressional setting) given that folding between the fault planes records shortening in

the east-west direction.

It is also possible that D3 is the product of the same dextral strike-slip regime. If

so, however, the obliquity of D4 structures to those of D3 requires explanation. With

respect to the orientation of the extension direction, D3 structures are rotated in a clockwise-rotated sense, which is consistent with dextral movement. If the development 93 of D3 structures during dextral strike-slip was followed by clockwise rotation during ongoing dextral movement that produced D4 structures, then the obliquity between D3 and D4 would be explained and two deformational phases would be linked geometrically.

Given that D2 is also associated with the dextral strike slip (S2-C fabrics), D2 through D4 may be regarded as the sequential products of a single progressive deformation.

The folds produced by the latest phase of deformation in the Granjeno Schist bear a similar style and orientation to those in the unmetamorphosed Silurian to Early Permian rocks that overlie the Novillo Gneiss (Stewart et al., 1999). If these folds are the product of the same deformational episode, then D4 must have been associated with little to no

metamorphism. Given that the folds in the unmetamorphosed succession are truncated

and unconformably overlain by Lower Jurassic rocks, the correlation would constrain D4 between the Early Permian and Lower Jurassic.

94 VII. Tectonic History of the Granjeno Schist

In order to develop a model for the evolution of the Granjeno Schist, several

factors must be considered, such as the time of accretion to the Novillo Gneiss, the

relationship of the Granjeno Schist to the leucogranite and the Paleozoic to Mesozoic

sedimentary successions, the types of deformation and metamorphism, and the tectonic

reconstructions that presently exist.

Tectonic reconstructions for the Paleozoic indicate that two major oceans existed

at different times between the Laurentian and Gondwanan landmasses, namely the

Iapetus and Rheic Oceans. Whereas the Iapetus Ocean achieved rift-drift transition in the

Cambrian (Cawood et al., 2001) and closed in the Late Ordovician-Early Silurian with

the accretion to Laurentia of continental arcs (e.g. the Carolina and Avalonia terranes;

Hibbard, 2000; Murphy et al., 1995), the rift-drift transition in the Rheic Ocean is

Ordovician in age, and coeval with the onset of Iapetus closure (Prigmore et al., 1997).

Ocean closure occurred in the Late Paleozoic with the amalgamation of Laurentia and

Gondwana to form Pangea (e.g. Alleghanian and Ouachita orogens; Hatcher, 2002).

Given that the Novillo Gneiss is unconformably overlain by Silurian strata of Gondwanan

affinity (Stewart et al., 1999), and that the newly acquired dates for the leucogranite and

for cooling following greenschist facies metamorphism in the Granjeno Schist suggest

Late Paleozoic events, correlation with the Rheic Ocean rather than the Iapetus Ocean is

favored.

Juxtaposing of the Granjeno Schist and Novillo Gneiss, and obduction of the

serpentinite-gabbro body, likely occurred during D1 prior to the intrusion of the leucogranite, since the latter is found between the Granjeno Schist and Novillo Gneiss, 95 and contains xenoliths of both units. In this scenario, the Granjeno Schist, the Novillo

Gneiss and its unmetamorphosed sedimentary cover formed part of the same margin prior to 354±54 Ma. Although clasts of Granjeno Schist provenance have been reported within the Silurian rocks unconformably overlying the Novillo Gneiss (Fries et al., 1962), suggesting an Early Paleozoic juxtaposition, this has not been confirmed by later detailed petrographic analysis (Stewart et al., 1999). Further constraints on the timing of D1 are presently unavailable.

D1 was followed by the intrusion of the leucogranite, which may have occurred in the same time interval as the growth of the post-D1 albite porphyroblasts. However, Ar-

Ar dates from the Novillo Gneiss do not document ages showing reheating at 354 Ma, suggesting that, in this area, the thermal effects of the leucogranite were minimal. Also, if significant movement occurred after the leucogranite intrusion, the Granjeno Schist and

Novillo Gneiss currently adjacent to the leucogranite would not have been next to it at ca.

354 Ma. The timing of intrusion (354±54 Ma) correlates within error to that of the El

Aserradero Rhyolite (dated at 334±39 Ma), which lies above the Lower Mississippian strata of the unmetamorphosed cover to the Novillo Gneiss, and may be the extrusive counterpart of the same igneous event. The Lower Mississippian unit underlying the rhyolite contains fossils of Laurentian affinity, so that intrusion of the leucogranite and rhyolitic volcanism must have occurred in proximity to North America. Hence, subsequent events likely record the final stages of amalgamation of Pangea.

D2 in the Granjeno Schist produced north-south folds correlative with dextral shear between the metamorphosed lithologies, the timing of which postdates the leucogranite intrusion. Muscovite from the intrusion gives an Ar-Ar age of 313±7 Ma, 96 which is taken to date cooling through 350°C (Purdy and Jäger, 1976). Phengite from the

S1/S2 composite foliation in quartz keratophyre yields a 350°C cooling age of 300±4 Ma,

likely constraining D2 deformation to the Late Carboniferous. The Mississippian to

Lower Pennsylvanian unconformity likely corresponds to D1 and/or D2.

D3 developed under chlorite to subgreenschist facies conditions with little new mineral growth, so it is likely that Ar-Ar cooling age of 300±4Ma for the quartz keratophyre predates D3, suggesting that D3 occurred during the Permian. In the sedimentary units above the Novillo Gneiss, the mid-Pennsylvanian-Lower Permian unconformity may be coeval with D3.

The last phase of penetrative deformation (D4) recorded in the Granjeno Schist may also have formed in the same dextral strike-slip regime as both D2 and D3. This would imply that deformation continued into the Permian, as evidenced by the deformed

Lower Permian forearc basin flysch deposits (Stewart et al., 1999), and the dextral and sinistral faults throughout the Granjeno Schist whose orientations are consistent with the strike-slip regime. This deformation must have ended by the Lower Jurassic, since the latter unconformably overlies all pre-Mesozoic units, and truncates the NNW-SSE faults of the dextral shear regime. 97 VIII. Potential linkages to the Acatlán Complex of southern Mexico

The Acatlán Complex of southern Mexico (Fig. 1.1) bears striking similarity to

the Granjeno Schist and its tectonically associated serpentinite-metagabbro body, in that

it includes metasedimentary and metavolcanic units, mafic-ultramafic assemblages, and

granitoids, which are tectonically juxtaposed against Grenville-aged gneisses of the

Oaxacan Complex, and have been interpreted as subduction-related units incorporated

into an accretionary prism (Ortega-Gutiérrez, 1993). Furthermore, the bordering Novillo

Gneiss and correlative Oaxaca Complex occur along dextral shear zones (Elías-Herrera

and Ortega-Gutiérrez, 2002), both of which are overlain by Lower Paleozoic strata of

Gondwanan affinity. These geologic parallels, and the broad similarities in

deformational ages (discussed below), support linkages between the two areas, and may

aid in the refinement of Paleozoic plate reconstructions.

The Granjeno Schist and associated units comprise a package of rocks suggestive

of an accretionary prism setting, or perhaps, behind the prism and within a forearc basin,

given the lack of widespread mélange typical of accretionary prisms. In either tectonic

setting, deformation must in part be directly or indirectly related to subduction/obduction

processes. The polarity of subduction at this time is unknown, yet, if the Granjeno Schist

and Novillo Gneiss are respectively related to the more intensely studied Acatlán and

Oaxacan Complexes of southern Mexico, then correlations can be proposed. As part of

their recent work in the Acatlán Complex, Nance et al. (2004) infer Early Carboniferous

westward-vergent thrusting of obducted oceanic and continental lithosphere (Acateco

Subgroup), the result of which produced D1 structures in the obducted slice and deformation and greenschist facies metamorphism in the underlying Petlalcingo 98 Subgroup. If the two areas do indeed represent geologically- and time-correlative units, then both may have evolved on the same ocean margin, and the D1 structures in the

Granjeno Schist may be likewise Early Carboniferous in age and the result of westward obduction. The minimum age for D1 in the Granjeno Schist based on the leucogranite intrusion (354±54 Ma) spans the interval Lower Devonian to Mid-Pennsylvanian, but embraces the age of D1 in the Acatlán Complex (346±3 Ma; Nance et al., 2004).

D2 structures in the Acatlán Complex are associated with north-south dextral transpression and south-vergent thrusting, and are closely followed by D3, which produced macroscopic north-south upright folds and a penetrative crenulation fabric

(Malone et al., 2002). D2 and D3, which developed under greenschist facies conditions, are constrained between 290 Ma and 280±4 – 269±7 Ma, and are both attributed to the collision of Gondwana and southern Laurentia. Late Paleozoic reconstructions of Pangea indicate that northward movement of Mexican blocks is required during the final stages of amalgamation, and thus north-south dextral shear east of Acatlán is consistent with the reconstructions. D2 in the Granjeno Schist may likewise be ca. 300 Ma in age, and

records uplift (or unroofing) and NNW-SSE dextral shear apparently along strike from

the dextral shear regime of the Acatlán Complex. These similarities in styles and timing

of deformation, and their positions along-strike from each other, may link the two areas

in the Late Paleozoic as vestiges of Rheic Ocean closure during the final stages of Pangea

amalgamation.

In contrast to the D4 deformation in the Granjeno Schist, subsequent deformation in the Acatlán Complex (i.e., post D2-D3) has not been recognized. However, the timing of D4, which is also attributed to ongoing dextral shear, may closely approximate that of 99 D2-D3 in the Acatlán Complex. Hence, D2-D4 in the Granjeno Schist and D2-D3 in the along-strike Acatlán Complex likely preserve related histories of deformation and metamorphism, during the closing of the Rheic Ocean (Figure 8.1). 100

Figure 8.1 Pangea reconstruction showing location of Granjeno Schist and Permo- Triassic arc in a Permo-Triassic reconstruction of North, Middle and South America (modified from Keppie et al., 2003). The Acatlán Complex and Granjeno Schist each border gneiss complexes of Grenville age along a dextral strike shear zone.

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