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At least 469 records · Page 26Linked to original sources
USGS1971

Remanent magnetization and susceptibility of late Cenozoic rocks from New Zealand

Measurements of magnetic remanence and susceptibility were made on 176 samples of volcanic rocks from 22 late Cenozoic formations from the North Island. Intensities of remanence, susceptibilities, and Q ratios are highly variable in both the ignimbrites and andesites and their ranges overlap, indicating that the magnetic anomalies produced by ignimbrite bodies would be difficult to distinguish from those produced by andesite flows. An analysis of the directions of magnetization of the ignimbrites indicates that groups of petrologically distinct ignimbrites probably formed during the same volcanic episode.

North Island↗
USGS2024

Detrital zircons and the magmatic history of Viti Levu, Fiji

We integrate the existing detrital zircon data from multiple modern river sediment samples on Viti Levu, Fiji, with the most current available geological and topographic mapping of the respective river drainage basins to compare detrital populations with potential bedrock sources. The temporal and spatial variations in zircon geochemistry supplement what is known from igneous rocks and confirm the petrological differences between plutonic and volcanic rocks from the Eocene to early Oligocene (Yavuna age, >30 Ma), middle Oligocene to middle Miocene (Wainimala age, 30–12.5 Ma), late Miocene (Colo age, 12.5–6.5 Ma) and latest Miocene (Namosi age, 6.5–5 Ma). The >30 Ma Yavuna-age zircons are restricted to areas that drain the previously mapped Yavuna Group. The 30–12.5 Ma zircons are found across central Viti Levu from west to east, and the 30–15 Ma zircons have distinctively low U/Yb and high Dy/Yb ratios. They are the best radiometric evidence of widespread early to middle Miocene arc magmatism in Fiji that was relatively U-poor. Peak deconvolution of the Colo age zircons from individual basins suggests the following ages for undated or poorly dated plutons from central Viti Levu. The large Mavuvu pluton is probably composed of multiple intrusions in the 12–10 Ma range, the Waiqa pluton is probably ca 10 Ma, and the Noikoro pluton is probably ca 9 Ma. There are zircons from unknown plutonic or volcanic sources between 8 and 7 Ma in western Viti Levu that have distinct Eu/Eu* ratios. We attribute the highest U/Yb ratios in some Colo age zircons to crustal anatexis. Namosi-age zircons are abundant in the Medrausucu Group and can be found scattered across Viti Levu.

Viti Levu↗
USGS2014

Science applications of a multispectral microscopic imager for the astrobiological exploration of Mars

Future astrobiological missions to Mars are likely to emphasize the use of rovers with in situ petrologic capabilities for selecting the best samples at a site for in situ analysis with onboard lab instruments or for caching for potential return to Earth. Such observations are central to an understanding of the potential for past habitable conditions at a site and for identifying samples most likely to harbor fossil biosignatures. The Multispectral Microscopic Imager (MMI) provides multispectral reflectance images of geological samples at the microscale, where each image pixel is composed of a visible/shortwave infrared spectrum ranging from 0.46 to 1.73 μ m. This spectral range enables the discrimination of a wide variety of rock-forming minerals, especially Fe-bearing phases, and the detection of hydrated minerals. The MMI advances beyond the capabilities of current microimagers on Mars by extending the spectral range into the infrared and increasing the number of spectral bands. The design employs multispectral light-emitting diodes and an uncooled indium gallium arsenide focal plane array to achieve a very low mass and high reliability. To better understand and demonstrate the capabilities of the MMI for future surface missions to Mars, we analyzed samples from Mars-relevant analog environments with the MMI. Results indicate that the MMI images faithfully resolve the fine-scale microtextural features of samples and provide important information to help constrain mineral composition. The use of spectral endmember mapping reveals the distribution of Fe-bearing minerals (including silicates and oxides) with high fidelity, along with the presence of hydrated minerals. MMI-based petrogenetic interpretations compare favorably with laboratory-based analyses, revealing the value of the MMI for future in situ rover-mediated astrobiological exploration of Mars.

Astrobiology↗
USGS2020

Mariana serpentinite mud volcanism exhumes subducted seamount materials: Implications for the origin of life

The subduction of seamounts and ridge features at convergent plate boundaries plays an important role in the deformation of the overriding plate and influences geochemical cycling and associated biological processes. Active serpentinization of forearc mantle and serpentinite mud volcanism on the Mariana forearc (between the trench and active volcanic arc) provides windows on subduction processes. Here, we present (1) the first observation of an extensive exposure of an undeformed Cretaceous seamount currently being subducted at the Mariana Trench inner slope; (2) vertical deformation of the forearc region related to subduction of Pacific Plate seamounts and thickened crust; (3) recovered Ocean Drilling Program and International Ocean Discovery Program cores of serpentinite mudflows that confirm exhumation of various Pacific Plate lithologies, including subducted reef limestone; (4) petrologic, geochemical and paleontological data from the cores that show that Pacific Plate seamount exhumation covers greater spatial and temporal extents; (5) the inference that microbial communities associated with serpentinite mud volcanism may also be exhumed from the subducted plate seafloor and/or seamounts; and (6) the implications for effects of these processes with regard to evolution of life.

Philosophical Transactions of the Royal Society A:↗
USGS2007

A three-dimensional geophysical model of the crust in the Barents Sea region: Model construction and basement characterization

BARENTS50, a new 3-D geophysical model of the crust in the Barents Sea Region has been developed by the University of Oslo, NORSAR and the U.S. Geological Survey. The target region comprises northern Norway and Finland, parts of the Kola Peninsula and the East European lowlands. Novaya Zemlya, the Kara Sea and Franz-Josef Land terminate the region to the east, while the Norwegian-Greenland Sea marks the western boundary. In total, 680 1-D seismic velocity profiles were compiled, mostly by sampling 2-D seismic velocity transects, from seismic refraction profiles. Seismic reflection data in the western Barents Sea were further used for density modelling and subsequent density-to-velocity conversion. Velocities from these profiles were binned into two sedimentary and three crystalline crustal layers. The first step of the compilation comprised the layer-wise interpolation of the velocities and thicknesses. Within the different geological provinces of the study region, linear relationships between the thickness of the sedimentary rocks and the thickness of the remaining crystalline crust are observed. We therefore, used the separately compiled (area-wide) sediment thickness data to adjust the total crystalline crustal thickness according to the total sedimentary thickness where no constraints from 1-D velocity profiles existed. The BARENTS50 model is based on an equidistant hexagonal grid with a node spacing of 50 km. The P -wave velocity model was used for gravity modelling to obtain 3-D density structure. A better fit to the observed gravity was achieved using a grid search algorithm which focussed on the density contrast of the sediment-basement interface. An improvement compared to older geophysical models is the high resolution of 50 km. Velocity transects through the 3-D model illustrate geological features of the European Arctic. The possible petrology of the crystalline basement in western and eastern Barents Sea is discussed on the basis of the observed seismic velocity structure. The BARENTS50 model is available at http://www.norsar.no/seismology/barents3d/ .

Geophysical Journal International↗
USGS2009

Constraints on the depth of generation and emplacement of a magmatic epidote-bearing quartz diorite pluton in the Coast Plutonic Complex, British Columbia

Petrology and P–T estimates indicate that a magmatic epidote-bearing quartz diorite pluton from Mt. Gamsby, Coast Plutonic Complex, British Columbia, was sourced at pressures below ∼1.4 GPa and cooled nearly isobarically at ∼0.9 GPa. The P–T path indicates that the magma was within the stability field of magmatic epidote early and remained there upon final crystallization. The pluton formed and crystallized at depths greater than ∼30 km. REE data indicate that garnet was absent in the melting region and did not fractionate during crystallization. This suggests that the crust was less than or equal to ∼55 km thick at 188 Ma during the early phases of magmatism in the Coast Plutonic Complex. Late Cretaceous contractional deformation and early Tertiary extension exhumed the rocks to upper crustal levels. Textures of magmatic epidote and other magmatic phases, combined with REE data, can be important for constraining the P–T path followed by magmas.

British Columbia↗
USGS2012

Evidence for a late thermal event of unequilibrated enstatite chondrites: a Rb-Sr study of Qingzhen and Yamato 6901 (EH3) and Khairpur (EL6)

The Rb-Sr whole rock and internal systematics of two EH3 chondrites, Qingzhen and Yamato 6901, and of one EL6 chondrite, Khairpur, were determined. Sulfides were separated using a stepwise dissolution technique. The mineral species in each fraction were estimated based on the chemical analyses of 12 major elements. The internal Rb-Sr systematics of the EH3 chondrites are highly disturbed. Fractions corresponding to sulfide phases show excess 87 Sr, while other fractions corresponding to silicate phases produce a linear trend on a Rb-Sr evolution diagram. If these linear relations are interpreted as isochrons, the ages of the silicate phases are 2.12 ± 0.23 Ga and 2.05 ± 0.33 Ga with the initial Sr isotopic ratios of 0.7112 ± 0.0018 and 0.7089 ± 0.0032, for Qingzhen and Yamato 6901, respectively. The process of the isotopic disturbance probably involved the breakdown of the major K-bearing sulfide (djerfisherite), and a lack of isotopic exchange between sulfide and silicate phases indicates moderate temperatures of reheating. Although a complete Sr isotopic re-homogenization among silicate phases was not attained, we interpret the Rb-Sr results as indicative of a late thermal event about 2 Ga ago on the parent bodies of these EH3 chondrites. These ages agree well with previously published K-Ar ages. An older isochron age of 4.481 ± 0.036 Ga with a low initial Sr isotopic ratio of 0.69866 ± 0.00038 was obtained for the data from silicate fractions of Khairpur, indicating early petrological equilibration on the parent body of EL6 chondrites.

Meteoritics↗
USGS2010

Spectral characteristics of iron-bearing phyllosilicates: Comparison to Orgueil (CI1), Murchison and Murray (CM2)

Phyllosilicate alteration minerals are commonly found in low petrologic types of carbonaceous chondrites. Previous spectral studies have examined Mg-bearing phyllosilicates with limited success in matching the spectral properties of CM and CI chondrites. Transmission electron microscope and other analytical techniques suggest that Fe-bearing clays are more abundant in CI and CM chondrites than magnesian varieties. Here, we present the results of an examination of the reflectance spectra of Fe-phyllosilicates, including serpentines and berthierines, of which the latter were formerly known as septechlorites. We have measured the diffuse reflectance spectra of powdered samples from 0.3 to 25 ??m. We find that these minerals provide a better spectral match to many of the features seen in CI and CM chondrites, and simple linear combinations of the spectra of both Fe- and Mg-phyllosilicates closely approximate the spectra of CM and CI chondrites.

Meteoritics and Planetary Science↗
USGS2010

Bleached chondrules: Evidence for widespread aqueous processes on the parent asteroids of ordinary chondrites

We present the first detailed study of a population of texturally distinct chondrules previously described by Kurat (1969), Christophe Michel-Lévy (1976), and Skinner et al. (1989) that are sharply depleted in alkalis and Al in their outer portions. These “bleached” chondrules, which are exclusively radial pyroxene and cryptocrystalline in texture, have porous outer zones where mesostasis has been lost. Bleached chondrules are present in all type 3 ordinary chondrites and are present in lower abundances in types 4–6. They are most abundant in the L and LL groups, apparently less common in H chondrites, and absent in enstatite chondrites. We used x-ray mapping and traditional electron microprobe techniques to characterize bleached chondrules in a cross section of ordinary chondrites. We studied bleached chondrules from Semarkona by ion microprobe for trace elements and H isotopes, and by transmission electron microscopy. Chondrule bleaching was the result of low-temperature alteration by aqueous fluids flowing through finegrained chondrite matrix prior to thermal metamorphism. During aqueous alteration, interstitial glass dissolved and was partially replaced by phyllosilicates, troilite was altered to pentlandite, but pyroxene was completely unaffected. Calcium-rich zones formed at the inner margins of the bleached zones, either as the result of the early stages of metamorphism or because of fluid-chondrule reaction. The mineralogy of bleached chondrules is extremely sensitive to thermal metamorphism in type 3 ordinary chondrites, and bleached zones provide a favorable location for the growth of metamorphic minerals in higher petrologic types. The ubiquitous presence of bleached chondrules in ordinary chondrites implies that they all experienced aqueous alteration early in their asteroidal histories, but there is no relationship between the degree of alteration and metamorphic grade. A correlation between the oxidation state of chondrite groups and their degree of aqueous alteration is consistent with the source of water being either accreted ices or water released during oxidation of organic matter. Ordinary chondrites were probably open systems after accretion, and aqueous fluids may have carried volatile elements with them during dehydration. Individual radial pyroxene and cryptocrystalline chondrules were certainly open systems in all chondrites that experienced aqueous alteration leading to bleaching.

Meteoritics and Planetary Science↗
USGS2013

Neutron absorption constraints on the composition of 4 Vesta

Global maps of the macroscopic thermal neutron absorption cross section of Vesta's regolith by the Gamma Ray and Neutron Detector (GRaND) on board the NASA Dawn spacecraft provide constraints on the abundance and distribution of Fe, Ca, Al, Mg, and other rock-forming elements. From a circular, polar low-altitude mapping orbit, GRaND sampled the regolith to decimeter depths with a spatial resolution of about 300 km. At this spatial scale, the variation in neutron absorption is about seven times lower than that of the Moon. The observed variation is consistent with the range of absorption for howardite whole-rock compositions, which further supports the connection between Vesta and the howardite, eucrite, and diogenite meteorites. We find a strong correlation between neutron absorption and the percentage of eucritic materials in howardites and polymict breccias, which enables petrologic mapping of Vesta's surface. The distribution of basaltic eucrite and diogenite determined from neutron absorption measurements is qualitatively similar to that indicated by visible and near infrared spectroscopy. The Rheasilvia basin and ejecta blanket has relatively low absorption, consistent with Mg-rich orthopyroxene. Based on a combination of Fe and neutron absorption measurements, olivine-rich lithologies are not detected on the spatial scales sampled by GRaND. The sensitivity of GRaND to the presence of mantle material is described and implications for the absence of an olivine signature are discussed. High absorption values found in Vesta's “dark” hemisphere, where exogenic hydrogen has accumulated, indicate that this region is richer in basaltic eucrite, representative of Vesta's ancient upper crust.

Meteoritics and Planetary Science↗
USGS2025

Rapid fault healing from cementation controls the dynamics of deep slow slip and tremor

Despite its status as one of the most important discoveries in geophysics, the physical mechanism(s) responsible for slow slip events (SSEs) are not well understood. Here, we synthesize observations of deep SSEs in the Cascadia Subduction Zone and argue that rapid, cohesive fault strengthening may control the dynamics of deep SSEs. Cohesive strength is frequently ignored in constitutive laws used to describe fault rheology in numerical simulations of earthquakes and SSEs alike. To demonstrate its importance, we perform and analyze a suite of petrological experiments that simulate fault healing under representative pressure and temperature conditions. We show that significant cohesive strength recovery caused by dissolution-precipitation processes occurs on timescales of just a few hours. Together, our experimental and observational results support the idea that cohesion is a key component of fault strength under SSE conditions and highlight the need for its inclusion in both future experiments and numerical models of fault slip.

British Columbia, Washington↗
USGS2008

Deep drilling into the Chesapeake Bay impact structure

Samples from a 1.76-kilometer-deep corehole drilled near the center of the late Eocene Chesapeake Bay impact structure (Virginia, USA) reveal its geologic, hydrologic, and biologic history. We conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact-melt creation and distribution, transient-cavity collapse, and ocean-water resurge. Comparison of post-impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long-term sedimentation patterns in the mid-Atlantic region. Salty connate water of the target remains in the crater fill today, where it poses a potential threat to the regional groundwater resource. Observed depth variations in microbial abundance indicate a complex history of impact-related thermal sterilization and habitat modification, and subsequent post-impact repopulation.

Science↗
USGS1981

Eastern Indian 3800-million-year-old crust and early mantle differentiation

Samarium-neodymium data for nine granitic and tonalite gneisses occurring as remnants within the Singhbhum granite batholith in eastern India define an isochron of age 3775 ± 89 × 10 6 years with an initial 143 Nd/ 144 Nd ratio of 0.50798 ± 0.00007. This age contrasts with the rubidium-strontium age of 3200 × 10 6 years for the same suite of rocks. On the basis of the new samarium-neodynium data, field data, and petrologic data, a scheme of evolution is proposed for the Archean crust in eastern India. The isotopic data provide evidence that parts of the earth's mantle were already differentiated with respect to the chondritic samarium-neodymium ratio 3800 × 10 6 years ago.

eastern India↗
USGS1981

Crustal processes of the Mid-Ocean Ridge

Independent geological and geophysical investigations of the Mid-Ocean Ridge system have begun to focus on the nature of the magma chamber system underlying its central axis. Thermal models predict the existence of a steady-state chamber beneath a thin crustal lid ranging in thickness from 2 to 13 kilometers. The only aspect of the system that these models fail to account for is the extremely slow spreading rates. Seismological studies reveal the existence of a low-velocity zone beneath segments of the East Pacific Rise, which is thought to correspond to a chamber system having a half-width of approximately 5 to 10 kilometers. These estimates compare favorably with those derived separately through petrological investigations of deep-sea drilling results, various sampling programs, and field and laboratory studies of ophiolites. The chamber is thought to be wing-shaped and to remain continuously open; it is thought to be fed from the center while simultaneously solidifying at the sides as spreading carries the two halves apart. Progressive fractionation occurs by crystal settling coupled with repeated replenishment and magma mixing in an open steady-state system. Near-bottom studies reveal that the zone of extrusion above the chamber is narrow, but its eruptive history is cyclic in nature, in conflict with the predictions of a steady-state model. On-bottom gravity data at 21°N on the East Pacific Rise reveal a negative gravity anomaly that may be related to the uppermost part of the chamber. The anomaly is only 2 kilometers wide and 1 kilometer below the sea floor. This feature may be associated with a short-term upper magma reservoir. The cyclic volcanic activity is directly related to the active phase of hydrothermal circulation responsible for the observed negative thermal anomaly. The volume of water associated with this circulation is equal to the entire ocean volume passing through the accretion zone approximately every 8 million years. This is about 0.5 percent of the world's rivers, but the effective transport rates of elements are comparable to those of rivers in that anomalies for individual elements are frequently between 100 and 1000 times the average river composition. The degree of subsurface dilution determines the final exit temperature and composition of the hydrothermal fluids, ranging from manganese domination at extreme dilution to iron at intermediate levels to sulfide deposition when low dilution occurs. The discovery of massive sulfide deposits on the East Pacific Rise is destined to have a profound impact on our understanding of ore-forming processes. Whether it will have any economic significance remains to be seen.

Science↗
USGS2000

Central San Juan caldera cluster: Regional volcanic framework

Eruption of at least 8800 km 3 of dacitic-rhyolitic magma as 9 major ash-slow sheets (individually 150-5000 km 3 ) was accompanied by recurrent caldera subsidence between 28.3 and about 26.5 Ma in the central San Juan Mountains, Colorado. Voluminous andesitic-decitic lavas and breccias were erupted from central volcanoes prior to the ash-flow eruptions, and similar lava eruptions continued within and adjacent to the calderas during the period of explosive volcanism, making the central San Juan caldera cluster an exceptional site for study of caldera-related volcanic processes. Exposed calderas vary in size from 10 to 75 km in maximum diameter, the largest calderas being associated with the most voluminous eruptions. After collapse of the giant La Garita caldera during eruption if the Fish Canyon Tuff at 17.6 Ma, seven additional explosive eruptions and calderas formed inside the La Garita depression within about 1 m.y. Because of the nested geometry, maximum loci of recurrently overlapping collapse events are inferred to have subsided as much as 10-17 km, far deeper than the roof of the composite subvolcanic batholith defined by gravity data, which represents solidified caldera-related magma bodies. Erosional dissection to depths of as much as 1.5 km, although insufficient to reach the subvolcanic batholith, has exposed diverse features of intracaldera ash-flow tuff and interleaved caldera-collapse landslide deposits that accumulated to multikilometer thickness within concurrently subsiding caldera structures. The calderas display a variety of postcollapse resurgent uplift structures, and caldera-forming events produced complex fault geometries that localized late mineralization, including the epithermal base- and precious-metal veins of the well-known Creede mining district. Most of the central San Juan calderas have been deeply eroded, and their identification is dependent on detailed geologic mapping. In contrast, the primary volcanic morphology of the symmetrically resurgent Creede caldera, the volcanic framework for Lake Creede, has been exceptionally preserved because of rapid infilling by moat sediments of the Creede Formation, which were preferentially eroded during the past few million years. The ash-flow tuffs and caldera of the central San Juan region have been widely recognized as exceptional sites for study of explosive volcanic processes, and the results reported here provide new insights into processes of pyroclastic eruption and emplacement, geometric interrelations between caldera subsidence and resurgence, the petrologic diversity of sequential ash-flow eruptions, recurrent eruption of intermediate-composition lavas after each caldera-forming event, associated regional fault development, volume relations between ash-flow eruptions and associated calderas, the emplacement of subvolcanic batholiths, and involvement of mantle-derived mafic phases in magma-generation processes.

Colorado↗
USGS2004

Age and evolution of the Precambrian crust of the Tobacco Root Mountains, Montana

U-Pb analyses of zircons from gneisses, anatectic leucosome, metasedimentary rocks, and a younger (metamorphosed) mafic dike from the Tobacco Root Mountains of southwestern Montana document a Precambrian history that extends from at least 3.90–1.77 Ga. The oldest U-Pb age reported here (3.8 Ga) is from a detrital zircon from a quartzite within the Spuhler Peak Metamorphic Suite, although younger ages of clearly detrital grains suggest the protolith was deposited subsequent to 3.2 Ga. Alternatively, a Pb-Pb age of ca. 2.45 Ga from a single subhedral zircon from this quartzite suggests the quartzite, and perhaps other Spuhler Peak Metamorphic Suite lithologies, may have formed in the Proterozoic. An Archean age, however, seems most compatible with the Archean Sm-Nd model ages of mafic and metasedimentary components of the Spuhler Peak Metamorphic Suite and the age distribution of zircons from the quartzite, which is very similar to the age distribution present in Archean quartzites in the region. The Spuhler Peak Metamorphic Suite lies in tectonic contact with volumetrically dominant, Archean, quartzofeldspathic gneisses and intercalated metasedimentary rocks. The protoliths of these gneisses were apparently emplaced 3.2–3.4 Ga, and are interpreted to be the basement upon which the intercalated (meta)sedimentary rocks were deposited. U-Pb analyses of zircons from anatectic leucosome near the boundary between the gneisses and the Spuhler Peak Metamorphic Suite, however, yield a significant population of 1.77 Ga grains, which are interpreted to have crystallized from the leucosome. All other grains are Archean (to 3.48 Ga) and interpreted to derive from the metasedimentary source of the leucosome. In addition, U-Pb analyses of zircons extracted from a granulite facies mafic dike that cuts across Archean gneissic banding indicate the dike was intruded at 2.06 Ga, but reached granulite facies at 1.76 Ga. Structural, petrologic, and geochronologic data suggest all lithologies experienced granulite facies metamorphism at ca. 1.77 Ga and that the Spuhler Peak Metamorphic Suite was tectonically emplaced after 2.06 Ga, but before 1.77 Ga. This Paleoproterozoic tectonic activity is most likely a result of burial during terrane collision (e.g., the juxtaposition of the Wyoming and Hearne provinces) and/or to postcollisional mafic underplating.

Montana↗
USGS2005

Contrasting Proterozoic basement complexes near the truncated margin of Laurentia, northwestern Sonora–Arizona international border region

We utilize new geological mapping, conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) and sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses, and whole-rock radiogenic isotope characteristics to distinguish two contrasting Proterozoic basement complexes in the international border region southeast of Yuma, Arizona. Strategically located near the truncated southwest margin of Laurentia, these Proterozoic exposures are separated by a northwest-striking Late Cretaceous batholith. Although both complexes contain strongly deformed Paleoproterozoic granitoids (augen gneisses) intruded into fine-grained host rocks, our work demonstrates marked differences in age, host rock composition, and structure between the two areas. The Western Complex reveals a >5-km-thick tilted section of finely banded felsic, intermediate, and mafic orthogneiss interspersed with tabular intrusive bodies of medium-grained leucocratic biotite granite (1696 ± 11 Ma; deepest level), medium-grained hornblende-biotite granodiorite (1722 ± 12 Ma), and coarse-grained porphyritic biotite granite (1725 ± 19 Ma; shallowest level). Penetrative ductile deformation has converted the granites to augen gneisses and caused isoclinal folding and transposition of primary contacts. Exposed in a belt of northwest-trending folds, these rocks preserve southwest-vergent shear fabric annealed during amphibolite facies metamorphism, when crystalloblastic textures developed. Deformation and regional metamorphism occurred before emplacement of 1.1 Ga(?) mafic dikes. Throughout the Eastern Complex, meta-arkose, quartzite, biotite schist, and possible felsic metavolcanic rocks comprise the country rocks of strongly foliated medium- and coarse-grained biotite granite augen gneisses that yield mean 207 Pb/ 206 Pb ages of 1646 ± 10 Ma, 1642 ± 19 Ma, and 1639 ± 15 Ma. Detrital zircons from four samples of host sandstone are isotopically disturbed; nevertheless, the data indicate a restricted provenance (ca. 1665 Ma to 1650 Ma), with two older grains (1697 and 1681 Ma). The pervasively recrystallized Paleoproterozoic map units strike parallel to foliation and are repeated in south-trending folds that are locally refolded about easterly hinges. Southeasterly lineation developed in augen gneiss and host strata becomes penetrative in local domains of L-tectonite. Regional metamorphism associated with this tectonism persisted until ca. 1590 Ma, as recorded by metamorphic growths within some zircon grains. Mesoproterozoic intrusions that crosscut the Paleoproterozoic metasediments and augen gneisses include coarsely porphyritic biotite granite (1432 ± 6 Ma) and diabase dikes (1.1 Ga?). Emplacement of the granite was accompanied by secondary high-U overgrowths, dated at 1433 ± 8 Ma, on some of the Paleoproterozoic detrital zircons, and apparently was also responsible for resetting the whole-rock Pb isotopic systematics (1441 ± 39 Ma) within these Eastern Complex augen gneisses. Younger plutons emplaced into both Proterozoic basement complexes include medium-grained quartz diorite (73.4 ± 3.3 Ma and 72.8 ± 1.7 Ma), Late Cretaceous hornblende-biotite granodiorite, and Paleogene leucocratic biotite granite. Neogene sedimentary and volcanic strata overlie basement along unconformities that are tilted to the northeast, southeast, or southwest. A brittle normal fault, dipping gently northeast, juxtaposes Tertiary andesite with Paleoproterozoic metasandstone. These relationships suggest that the area shares a common history of mid-Tertiary extension with southwestern Arizona. Later influence of the southern San Andreas fault system is implied by multiple dextral offsets of pre-Tertiary units across northwest-trending valleys. Our structural, geochronologic, and isotopic data provide new information to constrain pre–750 Ma Rodinia reconstructions involving southwestern Laurentia. Whole-rock U-Th-Pb and Rb-Sr isotopic systematics in both Paleoproterozoic gneiss complexes are disturbed, however, well-behaved Sm-Nd analyses preserve depleted initial ε Nd values (+2 to +4) that are distinct from the Mojave crustal province, but overlapping with the Yavapai and Mazatzal Provinces of Arizona. The Eastern Complex has the appropriate age and Nd isotopic signature to be part of the Mazatzal Province, but records major tectonism and metamorphism at ca. 1.6 Ga that postdates the Mazatzal orogeny. Deformed granitoids of the Western Complex have “Yavapai-type” ages and ε Nd but display structures discordant to the southwesterly Yavapai trend in central Arizona. The Western Complex lies along-strike with similar-age rocks (1.77 Ga to 1.69 Ga) of the “Caborca block” that have only been studied in detail near Quitovac and south of Caborca. Collectively, these rocks form a northwest-trending strip of basement situated at the truncated edge of Laurentia. The present-day basement geography may reflect an original oroclinal bend in the Yavapai orogenic belt. Alternatively, the western Proterozoic belt of Sonora may represent displaced fragments of basement juxtaposed against the Yavapai-Mazatzal Provinces along a younger sinistral transform fault (e.g., the Late Jurassic Mojave-Sonora megashear or the Permian Coahuila transform). Crustal blocks with these specific petrologic, geochronologic, and isotopic characteristics can be found in south-central and northeastern portions of the Australian Proterozoic basement, further supporting a connection between the two continents prior to breakup of the Rodinian supercontinent.

Arizona, California, Nevada, Sonora↗
USGS1969

Alkalic and tholeiitic basaltic volcanism related to the Rio Grande depression, southern Colorado and northern New Mexico

Upper Cenozoic basaltic rocks in and near the northern Rio Grande depression, a major intracontinental tension-rift structure, vary systematically in petrology and chemistry with distance from the depression. Basalts and basaltic andesites of alkalic affinities, commonly showing evidence of crustal contamination, were erupted east and west of the depression concurrently with its formation, whereas little-contaminated tholeiitic basalts filled parts of the depression late in its history. Eruption of the contrasting basalt types was in part concurrent. The lateral change from alkalic to tholeiitic basaltic volcanism may reflect different conditions of magma generation in the mantle that are related to changes in crustal thickness and thermal gradient across the rift. Recent experimental studies suggest that the variations in magma composition may be due to differing depths of magma fractionation, the tholeiitic basalts originating at shallower depths than the alkalic basalts.

Colorado, New Mexico↗