Gordon B. Haxel

Late Triassic paleogeography of the southern Cordillera: The problem of a source for voluminous volcanic detritus in the Chinle Formation of the Colorado Plateau region

The Upper Triassic Chinle Formation of the Colorado Plateau contains voluminous volcanic detritus evidently derived from a source to the south. Volcanic rocks exposed in southern Arizona and northern Sonora have been assumed to represent this source terrane, but U-Pb isotopic geochronology and regional stratigraphic correlations indicate that these volcanic rocks are distinctly younger than the Chinle, and thus not a source for the volcanic detritus in the Chinle. Igneous rocks of known or possible Late Triassic age in Nevada, California, or northeastern Mexico are possible sources, but a clearly defined source terrane for the volcanic detritus in the Chinle has not been identified. Tectonic removal of the source terrane by rifting or strike-slip offset, though not proven, is a possibility.

Subduction and exhumation of the Pelona-Orocopia-Rand schists, southern California

The Pelona, Orocopia, and Rand schists of southern California and southwestern Arizona are thought to have formed in either the same east-dipping subduction zone as the Franciscan complex or in a southwest-dipping subduction zone related to collision of an outboard continental fragment with North America. The principal justification for the collision model has been the observation that continental rocks overlying the schists locally show transport to the northeast. Field and petrographic studies now confirm that the northeast movement occurred during exhumation of the schist, not during subduction. Combined with regional geologic relations, this evidence makes the collision model unlikely.

Latest Cretaceous and early Tertiary orogenesis in south-central Arizona: Thrust faulting, regional metamorphism, and granitic plutonism

Rocks in a number of mountain ranges in southernmost central Arizona are juxtaposed by thrust faults, regionally metamorphosed, and intruded by garnet–two-mica granites. Field relations and K-Ar and U-Pb isotopic geochronology indicate that thrust faulting, metamorphism, and granitic plutonism were closely related aspects of a latest Cretaceous and early Tertiary orogenic episode. The regionally metamorphosed rocks, chiefly greenschist-facies quartzofeldspathic schists, were derived from Jurassic and Cretaceous sedimentary, volcanic, and plutonic rocks. In five ranges, these metamorphic rocks are overlain, along synmetamorphic thrust faults, by Precambrian gneiss or Late Jurassic or Late Cretaceous plutonic rocks. In three additional ranges, upward increase of textural grade within the metamorphic rocks and analogy with the areas of exposed thrust faults indicate the likelihood of a concealed thrust fault flanking the range. Seven of these eight thrust faults have single lower plates, but one thrust is underlain by a duplex consisting of several imbricate structural sheets separated by mylonitic tectonic slides. Areal structural relations and the regional arrangement of distinctive Jurassic igneous rock units strongly suggest that one large range, the Baboquivari Mountains, and several smaller ranges are fensters in a thrust-fault system of regional extent, and that several adjacent ranges are allochthonous. The regionally metamorphosed rocks are intruded by synmetamorphic to postmetamorphic early Tertiary leucocratic granites characterized by various combinations of accessory biotite, muscovite, and garnet. At least two of these granites contain inherited Precambrian zircon indicative of generation by crustal anatexis. K-Ar and(or) U-Pb isotopic ages of premetamorphic granites, metamorphic rocks, thrust-zone mylonites, synmetamorphic granites, and metamorphogenic fissure veins indicate that the orogenic episode commenced in Late Cretaceous time and culminated in early Tertiary time, 58 to 60 m.y. ago. The observed and inferred close spatial and temporal relations between thrust faulting, regional metamorphism, and granitic plutonism lead to the following hypothesis for orogenesis in south-central Arizona: crustal compression caused overthrusting of crystalline rocks, resulting in crustal thickening; and crustal thickening and conductive and magmatic heat flux from the mantle together set up a thermal regime within which regional metamorphism and the generation and emplacement of the anatectic granites took place.

A garnet-two-mica granite, Coyote Mountains, southern Arizona: Geologic setting, uranium-lead isotopic systematics of zircon, and nature of the granite source region

The 58-m.y.-old Pan Tak Granite, considered representative of the widespread early Tertiary garnet-two-mica granites of south-central Arizona, consists of biotite and muscovite-biotite granite intruded by garnet-muscovite and garnet-biotite granite and associated pegmatite. Textural evidence indicates that muscovite is a primary, magmatic mineral. The granite was intruded into Jurassic dioritic to granitic rocks and Paleozoic metasedimentary rocks during a latest Cretaceous and early Tertiary regional metamorphic episode. Part of the pluton was subsequently mylonitized during an early or middle Tertiary deformational episode of “metamorphic-core-complex” aspect. The fabrics produced by these two metamorphic episodes differ in distribution, style, orientation, and, probably, age. The zircon population of the Pan Tak Granite consists of a mixture of small euhedral crystals and larger, rounded crystals with or without overgrowths. Uranium-lead isotopic ratios of five size fractions of discordant zircon define a chord with a lower concordia intercept age of 58 ± 2 m.y. and a late Precambrian upper intercept. 207 Pb*/ 206 Pb* ages range from 214 m.y. for the finest fraction to 434 m.y. for the coarsest fraction. The Pan Tak discordia is interpreted as a mixing line between 58-m.y.-old magmatic zircon and an older zircon component largely or entirely of late Precambrian age. Geologic and isotopic evidence indicates that this older zircon component was not assimilated from the country rocks but, rather, was inherited from the source material of the granite. The presence of this inherited zircon indicates that the Pan Tak magma was produced by melting of crustal rocks containing late Precambrian zircon. This source material was probably the 1.7- to 1.4-b.y.-old crystalline basement of south-central Arizona. Generation of the Pan Tak and related granites by crustal anatexis was one aspect of a latest Cretaceous to early Tertiary orogenic episode in south-central Arizona.

Exhumation history of the Orocopia Schist and related rocks in the Gavilan Hills area of southeasternmost California

The Gavilan Hills area of southeasternmost California exposes three distinctly different crystalline rock packages in a postmetamorphic, E–W elongated dome. Structurally deepest is the relatively high-pressure, eugeoclinal Orocopia Schist, which underlies the low-angle Chocolate Mountains fault. Above the schist are gneisses derived from midto lower-crustal levels of the Mesozoic Cordilleran magmatic arc. The gneisses, in turn, are separated by the Gatuna fault from low-grade metasedimentary and metavolcanic rocks of the Winterhaven Formation. The Chocolate Mountains fault was originally thought to be a SW-dipping subduction thrust along which an exotic continental sliver was sutured to North America. Recent workers, however, have proposed that it is a late fault responsible for exhumation of the Orocopia Schist and that it places no constraints on burial history. The latter interpretation is supported by the presence in the schist of two distinct structural fabrics, an older one presumably related to underthrusting, and a younger one attributed to exhumation. The older fabric is preserved in schist away from the Chocolate Mountains fault and is associated with a NNE–SSW-trending lineation that formed during prograde metamorphism to lowermost amphibolite facies. The younger fabric is best developed within 100 m structurally of the Chocolate Mountains fault and is characterized by discrete shear zones, greenschist-facies retrogression, and E–W-trending lineations. Lineations with similar orientation also occur in gneiss adjacent to both the Chocolate Mountains and Gatuna faults and in the Winterhaven Formation. This Jacobson, C.E., Grove, M., Stamp, M. M., Vucic, A., Oyarzabal, F.R., Haxel, G.B., Tosdal, R.M., and Sherrod, D.R., 2002, Exhumation history of the Orocopia Schist and related rocks in the Gavilan Hills area of southeasternmost California, in Barth, A., ed., Contributions to Crustal Evolution of the Southwestern United States: Boulder, Colorado, Geological Society of America Special Paper 365, p. 129–154. *E-mail: cejac@iastate.edu C.E. Jacobson et al. 130 observation, combined with interpretation of outcrop patterns, suggests that the Gatuna fault, which was previously considered a steep, shallow-level fault of Miocene age, is a low-angle structure that accommodated relatively high-temperature deformation similar to that recorded by the Chocolate Mountains fault. Both faults may have been synchronously active. However, because the Gatuna fault exhibits more intense brittle overprinting of early mylonitic fabrics and greater structural excision than the Chocolate Mountains fault, it is indicated to have been more recently active and the more important of the two in exhuming the schist and overriding gneiss to shallow crustal levels. Thermal history results based upon Ar/Ar analysis of hornblende, muscovite, biotite, and K-feldspar and previous apatite fission track measurements reveal a twostage exhumation history that we relate to slip along the Chocolate Mountains and Gatuna faults, respectively. An initial phase of rapid cooling occurred from 60 Ma to 44 Ma. Younger and more discordant Ar/Ar ages recorded by hornblendes from the schist (52–57 Ma) relative to the gneiss (59–64 Ma) confirm postpeak metamorphic juxtaposition of the two units along the Chocolate Mountains fault in a manner consistent with normal faulting. Coincidence of muscovite Ar/Ar ages between the Orocopia Schist and gneiss implies that this juxtaposition occurred by 48 2 Ma and indicate that the Chocolate Mountains fault is a Laramide-age structure. Biotite ages ranging from 45 to 31 Ma reveal that the initial exhumation phase was followed by protracted residence of the schist and gneiss in the middle crust at 350 C. Kfeldspars record a second period of rapid exhumation from 28 to 24 Ma, which we correlate with the brittle phase of movement on the Gatuna fault. This second phase of exhumation is considered to reflect an early stage of the middle Tertiary extensional event that is widespread in southeastern California and southwestern Arizona. Localized disruption of the Chocolate Mountains fault that juxtaposed structurally deeper schist against gneiss at the eastern end of the Gavilan Hills probably also occurred at this time.

Mantle peridotite in newly discovered far-inland subduction complex, southwest Arizona: initial report

The latest Cretaceous to early Palaeogene Orocopia Schist and related units are generally considered a low-angle subduction complex that underlies much of southern California and Arizona. A recently discovered exposure of Orocopia Schist at Cemetery Ridge west of Phoenix, Arizona, lies exceptionally far inland from the continental margin. Unexpectedly, this body of Orocopia Schist contains numerous blocks, as large as ~300 m, of variably serpentinized mantle peridotite. These are unique; elsewhere in the Orocopia and related schists, peridotite is rare and completely serpentinized. Peridotite and metaperidotite at Cemetery Ridge are of three principal types: (1) serpentinite and tremolite serpentinite, derived from dunite; (2) partially serpentinized harzburgite and olivine orthopyroxenite (collectively, harzburgite); and (3) granoblastic or schistose metasomatic rocks, derived from serpentinite, made largely of actinolite, calcic plagioclase, hercynite, and chlorite. In the serpentinite, paucity of relict olivine, relatively abundant magnetite (5%), and elevated Fe3+/Fe indicate advanced serpentinization. Harzburgite contains abundant orthopyroxene, only slightly serpentinized, and minor to moderate (1–15%) relict olivine. Mantle tectonite fabric is locally preserved. Several petrographic and geochemical characteristics of the peridotite at Cemetery Ridge are ambiguously similar to either abyssal or mantle-wedge (suprasubduction) peridotites and serpentinites. Least ambiguous are orthopyroxene compositions. Orthopyroxene is distinctively depleted in Al2O3, Cr2O3, and CaO, indicating mantle-wedge affinities. Initial interpretation of field and petrologic data suggests that the peridotite blocks in the Orocopia Schist subduction complex at Cemetery Ridge may be derived from the leading corner or edge of a mantle wedge, presumably in (pre-San Andreas fault) southwest California. However, derivation from a subducting plate is not precluded.

Extensional reactivation of the Chocolate Mountains subduction thrust in the Gavilan Hills of southeastern California

The NE vergent Chocolate Mountains fault of south-eastern California has been interpreted as either a subduction thrust responsible for burial and prograde metamorphism of the ensimatic Orocopia Schist or as a normal fault involved in the exhumation of the schist. Our detailed structural analysis in the Gavilan Hills area provides new evidence to confirm the latter view. A zone of deformation is present at the top of the Orocopia Schist in which lineations are parallel to those in the upper plate of the Chocolate Mountains fault but oblique to ones at relatively deep levels in the schist. Both the Orocopia Schist and upper plate contain several generations of shear zones that show a transition from crystalloblastic through mylonitic to cataclastic textures. These structures formed during retrograde metamorphism and are considered to record the exhumation of the Orocopia Schist during early Tertiary time as a result of subduction return flow. The Gatuna fault, which places low-grade, supracrustal metasediments of the Winterhaven Formation above the gneisses of the upper plate, also seems to have been active at this time. Final unroofing of the Orocopia Schist occurred during early to middle Miocene regional extension and may have involved a second phase of movement on the Gatuna fault. Formation of the Chocolate Mountains fault during exhumation indicates that its top-to-the-NE sense of movement provides no constraint on the polarity of the Orocopia Schist subduction zone. This weakens the case for a previous model involving SW dipping subduction, while providing support for the view that the Orocopia Schist is a correlative of the Franciscan Complex.

Middle Jurassic Topawa Group, Baboquivari Mountains, south-central Arizona: Volcanic and sedimentary record of deep basins within the Jurassic magmatic arc

Among supracrustal sequences of the Jurassic magmatic arc of the southwestern Cordillera, the Middle Jurassic Topawa Group, Baboquivari Mountains, south-central Arizona, is remarkable for its lithologic diversity and substantial stratigraphic thickness, ≈8 km. The Topawa Group comprises four units (in order of decreasing age): (1) Ali Molina Formation—largely pyroclastic rhyolite with interlayered eolian and fl uvial arenite, and overlying conglomerate and sandstone; (2) Pitoikam Formation—conglomerate, sedimentary breccia, and sandstone overlain by interbedded siltstone and sandstone; (3) Mulberry Wash Formation—rhyolite lava fl ows, fl ow breccias, and mass-fl ow breccias, with intercalated intraformational conglomerate, sedimentary breccia, and sandstone, plus sparse within-plate alkali basalt and comendite in the upper part; and (4) Tinaja Spring Porphyry—intrusive rhyolite. The Mulberry Wash alkali basalt and comendite are genetically unrelated to the dominant calcalkaline rhyolite. U-Pb isotopic analyses of zircon from volcanic and intrusive rocks indicate the Topawa Group, despite its considerable thickness, represents only several million years of Middle Jurassic time, between approximately 170 and 165 Ma. Sedimentary rocks of the Topawa Group record mixing of detritus from a minimum of three sources: a dominant local source of porphyritic silicic volcanic and Haxel, G.B., Wright, J.E., Riggs, N.R., Tosdal, R.M., and May, D.J., 2005, Middle Jurassic Topawa Group, Baboquivari Mountains, south-central Arizona: Volcanic and sedimentary record of deep basins within the Jurassic magmatic arc, in Anderson, T.H., Nourse, J.A., McKee, J.W., and Steiner, M.B., eds., The Mojave-Sonora megashear hypothesis: Development, assessment, and alternatives: Geological Society of America Special Paper 393, p. 329–357. doi: 10.1130/2005.2393(12). For permission to copy, contact editing@geosociety.org. ©2005 Geological Society of America. 330 G.B. Haxel et al.

A two-stage fluid history for the Orocopia Schist and associated rocks related to flat subduction and exhumation, southeastern California

ABSTRACT Stable isotopes combined with pre-existing 40Ar/39Ar thermochronology at the Gavilan Hills and Orocopia Mountains in southeastern California record two stages of fluid–rock interaction: (1) Stage 1 is related to prograde metamorphism as Orocopia Schist was accreted to the base of the crust during late Cretaceous–early Cenozoic Laramide flat subduction. (2) Stage 2 affected the Orocopia Schist and is related to middle Cenozoic exhumation along detachment faults. There is no local evidence that schist-derived fluids infiltrated structurally overlying continental rocks. Mineral δ18O values from Orocopia Schist in the lower plate of the Chocolate Mountains fault and Gatuna normal fault in the Gavilan Hills are in equilibrium at 490–580°C with metamorphic water (δ18O = 7–11‰). Phengite and biotite δD values from the Orocopia Schist and upper plate suggest metamorphic fluids (δD ~ –40‰). In contrast, final exhumation of the schist along the Orocopia Mountains detachment fault (OMDF) in the Orocopia Mountains was associated with alteration of prograde biotite and amphibole to chlorite (T ~ 350–400°C) and the influx of meteoric-hydrothermal fluids at 24–20 Ma. Phengites from a thin mylonite zone at the top of the Orocopia Schist and alteration chlorites have the lowest fluid δD values, suggesting that these faults were an enhanced zone of meteoric fluid (δD < –70‰) circulation. Variable δD values in Orocopia Schist from structurally lower chlorite and biotite zones indicate a lesser degree of interaction with meteoric-hydrothermal fluids. High fluid δ18O values (6–12‰) indicate low water–rock ratios for the OMDF. A steep thermal gradient developed across the OMDF at the onset of middle Cenozoic slip likely drove a more vigorous hydrothermal system within the Orocopia Mountains relative to the equivalent age Gatuna fault in the Gavilan Hills.