Richard M. Tosdal

The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Peru form the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171±20 Ma and 1158±12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (∼1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Belen Schist in northern Chile generally have present-day low 206Pb/204Pb ( 15.57), and elevated 208Pb/204Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher present-day 206Pb/204Pb values than those of the Early Proterozoic rocks of the Arequipa massif (206Pb/204Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton (206Pb/204Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link between these cratons and seemingly precluding part of the Arequipa-Antofalla craton from being a detached fragment of another craton such as eastern Laurentia, which has been characterized by a different U/Pb history. Pb isotopic compositions for the Arequipa-Antofalla craton are, furthermore, distinct from those of the Proterozoic basement in the Precordillera terrane, western Argentina, indicating a Pb isotopic and presumably a tectonic boundary between them. The Pb isotopic compositions for the Precordillera basement are similar to those of eastern Laurentia, and support other data indicating that these rocks are a detached fragment of North America. Finally, the distinct Pb isotopic evolution history of the Arequipa-Antofalla craton and eastern Laurentia require minor modification to tectonic models linking eastern North America-Scotland to the oroclinal bend in western South America.

El Salvador, Chile Porphyry Copper Deposit Revisited: Geologic and Geochronologic Framework

The Eocene (42 to 41 Ma) El Salvador porphyry copper deposit in the Indio Muerto district, northern Chile (26° 15′ S Lat.), formerly thought to have formed at the culmination of a 9-m.y. period of episodic magmatism, is shown by new mapping, U-Pb and K-Ar geochronology, and petrologic data to have formed during the younger of two distinct but superposed magmatic events-a Paleocene (∼63 to 58 Ma) and an Eocene (44 to 41 Ma) event. In the district, high-K Paleocene volcano-plutonic activity was characterized by a variety of eruptive styles and magmatic compositions, including a collapse caldera associated with explosive rhyolitic magmatism (El Salvador trapdoor caldera), a post-collapse rhyolite dome field (Cerro Indio Muerto), and andesitic-trachyandesitic stratovolcanos (Kilometro Catorce-Los Amarillos sequence). Pre-caldera basement faults were reactivated during Paleocene volcanism as part of the collapse margin of the caldera. Beneath Cerro Indio Muerto, where the porphyry Cu deposit subsequently forme...

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.

Crustal contamination in the petrogenesis of a calc-alkalic rock series: Josephine Mountain intrusion, California

The Josephine Mountain intrusion is a Cretaceous calc-alkalictonalite-granite pluton emplaced at 22 km depth in a continental margin arc. Variable uplift of adjacent terranes in southern California since mid-Cretaceous time allows us to reconstruct the local crustal column and evaluate its role as a contaminant of mantle-derived arc magmas in this region. The parental magma of the intrusion was high-alumina basalt whose isotopic signature ( 87 Sr/ 86 Sr = 0.7087; δ 18 O = 7.5; ϵ Nd = −10) cannot have been generated by intracrustal assimilation of known or inferred rock types in the middle or lower crust. Such a signature could have resulted from high-pressure fractionation of primary low-alumina basalt coupled with assimilation of felsic/pelitic lower crust, partial melting of enriched subcontinental mantle followed by high-pressure fractionation, or a combination of these processes. Tonalite of the intrusion was formed by fractionation of the parent magma coupled with assimilation of local felsic wall rocks or by crustal melts similar to slightly younger granite. Assessment of the magnitude of crustal contamination is hampered by uncertainty regarding the existence and role of partial melting of previously enriched subcontinental mantle in generating the parental basaltic magma, leading to concomitant uncertainty in the fraction of new continental crust created by such arc plutonism.

Geologic setting and tertiary structural evolution of southwestern Arizona and southeastern California

Volcanic and sedimentary rocks and structures record the Tertiary structural evolution of the lower Colorado River region in southwestern Arizona and southeastern California. A late Eocene or early Oligocene (prior to ∼33 Ma) episode of faulting is indicated by medium- to coarse-grained arkosic rocks in the Chocolate and southern Trigo Mountains. Much of the area was relatively quiescent tectonically during extrusion of volcanic rocks from ∼33 to 22 Ma, but the southernmost part was periodically uplifted and eroded into its underlying crystalline rocks. A major episode of extensional deformation and tilting occurred after deposition of welded tuff about 22 Ma and affected the entire area from the Palo Verde Mountains on the west to the Kofa Mountains on the east. Extension-related faulting quickly waned and had largely ceased by about 20 Ma in the Kofa Mountains; elsewhere the timing is poorly constrained. By ∼13 Ma, thick alluvial fans filled many grabens and half grabens among tilted fault blocks throughout the area. Volcanism in the lower Colorado River region may have been coincident with a broad structural depression now oriented east-west. The northern limit of the volcanic terrane defines a tilt-domain boundary. The northern boundary, reaching from the New Water Mountains in Arizona to the little Chuckwalla Mountains in California, ultimately evolved to separate a terrane of relatively untilted crystalline horsts on the north from a series of east or northeast dipping fault blocks on the south. The southern margin is less well defined but is subparallel to the northern boundary and to the Chocolate Mountains anticlinorium.

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.

Stratigraphic relations and U-Pb geochronology of the Upper Cretaceous upper McCoy Mountains Formation, southwestern Arizona

A previously unrecognized angular unconformity divides the Jurassic and Cretaceous McCoy Mountains Formation into a lower and an upper unit in the Dome Rock Mountains and Livingston Hills of western Arizona. The lower unit of the McCoy Mountains Formation consists of generally fine-grained quartzose and volcaniclastic strata that were deposited after cessation of Middle Jurassic explosive volcanism. The basal contact of the lower unit is disconformable in most places, but locally it has been interpreted to be gradational with the underlying silicic volcanic rocks. The upper unit is a fining-upward sequence of quartzo-feldspathic and arkosic conglomerate and sandstone that records uplift of a northern source terrane. A tuff in the lower part of the upper unit has a U-Pb crystallization age of 79 ± 2 Ma (Late Cretaceous). Rocks of the lower unit are deformed by pre-80 Ma thrust faults of the generally southward-vergent Maria fold and thrust belt, which bounds the outcrop belt of the McCoy Mountains Formation on the north. The upper unit is exposed only south of the fold and thrust belt. We interpret the intraformation unconformity in the McCoy Mountains Formation to have developed where rocks of the lower unit were deformed adjacent to the southern margin of the Maria fold and thrust belt. The upper unit of the formation is interpreted as a foreland-basin deposit that was shed southward from the actively rising and deforming fold and thrust belt. The apparent absence of an equivalent unconformity in the McCoy Mountains Formation in adjacent California is presumably a consequence of the observed westward divergence of the outcrop belt from the fold and thrust belt. Continued southward shortening deformed the entire formation under greenschist- and, locally, amphibolite-facies conditions soon after the upper unit was deposited. Tectonic burial beneath the north-vergent Mule Mountains thrust system in the latest Late Cretaceous (∼70 Ma) marked the end of Mesozoic contractile deformation in the area.

U-Pb ages of granitoid clasts in upper Mesozoic arc-derived strata of the Vizcaino Peninsula, Baja California, Mexico

Minor but widespread granitoid clasts occur in Late Jurassic to Early Cretaceous age volcanogenic strata of the Eugenia and Perforada Formations on the Vizcaino Peninsula of Baja California. Most are of two common types: coarse-grained garnet-bearing biotite granite and xenolith-rich hornblende tonalite. Concordia modeling of U-Pb zircon data from a biotite granite clast indicates crystallization at 150 +/- 3 Ma and a strong inherited component of Precambrian radiogenic Pb of 1.34 +/- 0.08 Ga. Data from a hornblende tonalite clast indicate a slightly younger crystallization age and a small component of inherited radiogenic Pb. These clasts occur in association with quartzose sediments that contain abundant biotite probably derived from the same source as the biotite granite. This nonvolcanic component occurs in significant amounts throughout most of the Tithonian-Valanginian Eugenia Formation and in the overlying Aptian-Albian Perforada Formation. The apparent absence of a nearby suitable plutonic source for the granitic detritus, either from the Baja California Peninsula or from Sonora, Mexico, may reflect translation of the Vizcaino Peninsula relative to the North American craton since Aptian-Albian time and prior to the opening of the Gulf of California at 5 Ma.

Geochemical studies in the Indian Pass and Picacho Peak Bureau of Land Management Wilderness study areas, imperial county, Southern California

Abstract The U.S. Geological Survey has conducted geochemical studies in the Indian Pass (CDCA-355), 124 km 2 , and Picacho Peak (CDCA-355A), 23 km 2 , Wilderness Study Areas (WSAs) as part of a program to evaluate the mineral resource potential of designated areas in the California Desert Conservation Area. These two WSAs are of particular interest because they lie within a region which has intermittently produced significant quantities of Au since the mid-1800s, and is currently the site of much exploration activity for additional Au resources. Within a 15-km radius of the WSAs, there is one actively producing gold mine, a major deposit which began production in 1986, and one recently announced discovery. In the reconnaissance geochemical surveys of the two WSAs - 177 μm (-80 mesh) stream sediments, heavy-mineral concentrates from stream sediments, and rocks were prepared and analyzed. Four areas of possible exploration interest were identified within the WSAs. The first area is characterized by anomalous W and Bi in nonmagnetic heavy-mineral concentrates, and is underlain primarily by the Mesozoic Orocopia Schist which has been intruded by monzogranite of Oligocene age. Alteration and mineralization appear to be localized near the intrusive contact. The mineralized rock at the surface contains secondary Cu and Fe minerals where the monzogranite intrudes the metabasite horizons of the Orocopia Schist and scheelite where the monzogranite intrudes marble within the Orocopia Schist. The second area is characterized by anomalous As, Sb, Ba, B, and Sr in nonmagnetic heavy-mineral concentrates and by anomalous As in - 177 μm stream sediments. Geologically, this area is underlain by metasedimentary and metavolcanic rocks of Jurassic(?) age; a biotite monzogranite of Jurassic(?) age; and Tertiary volcanic and hypabyssal rocks composed of flows, domes, and tuffs of intermediate to silicic composition. All these rock types are cut by a set of north-south-striking normal faults. The anomalies in the heavy-mineral concentrates are believed to be related to silica-clay alteration observed in the vicinity of some of these faults.

Gossans and Leached Capping; Field Assessment

Gossans and Leached Capping; Field Assessment. Roger Taylor. Pp. 146. 2011. Springer-Verlag. ISBN 978-3-642-22050-0. Price US