Avalonia of the northern Appalachian orogen includes clastic sedimentary rocks that span the Ediacaran-Cambrian boundary, including the Global Boundary Stratotype Section and Point (GSSP) at Fortune Head, Newfoundland. Detrital muscovite becomes abundant in sedimentary strata near and above the GSSP. Single-crystal 40Ar/39Ar total-fusion analyses for detrital muscovite sampled from these strata in Newfoundland and the Mira terrane of Nova Scotia yield ages that range from ca. 634 to 540 Ma, dominated by single modes that are skewed to younger ages. Laser incremental heating 40Ar/39Ar age spectra for individual muscovite crystals are typical of 40Ar loss, with ages for initial increments as young as ca. 400 Ma that generally increase through the measurements, defining plateau ages with an average of 637.1 +/- 4.7 Ma. Wavelength dispersive spectrometer maps show K and Al loss selectively along interlayer (001) zones and grain boundaries now occupied by aluminous, metamorphic chlorite. Loss of K and 40Ar was linked to regional, low-grade metamorphism during late Ediacaran transpression, overprinted by metamorphism during late Silurian-Devonian juxtaposition of Avalonia with Ganderia and Meguma. The detrital muscovite source is inferred to have been aluminous rocks in the Avalonian crust metamorphosed during ca. 650 collision of Avalonia with the West African Craton. The oldest muscovite ages coincide with the end of the Marinoan glacial stage, which could indicate a causal relationship between the termination of glaciation and initial mid-crustal cooling of the source terrane through closure temperatures for muscovite. Exhumation and erosion during extension and basin development through the Ediacaran-Cambrian transition provided abundant detrital muscovite.

Late Paleozoic strata are well preserved in numerous intracratonic basins on the Indian subcontinent, which was once part of East Gondwana. The sources and transport pathways of late Paleozoic sediments in these Gondwanan basins are poorly understood, owing primarily to the lack of geochronological data on detrital sediments. To reconstruct the regional detrital and tectonic history of East Gondwana, we have incorporated multiproxy provenance analyses that includes detrital 40Ar/39Ar geochronology, sandstone petrofacies, and heavy-mineral analyses of late Paleozoic strata of the Barapukuria, Dighipara, and Khalaspir basins of northwestern Bangladesh and the Jharia Basin of east-central India. We present the first detrital thermochronology data from the Jharia, Barapukuria, and Dighipara basins. Sandstone petrographic analysis indicates temporal and spatial variations in the compositions among adjacent Gondwanan basins. Heavy-mineral assemblages suggest amphibolite-and granulite-facies source terranes. Detrital 40Ar/39Ar data from the Barapukuria and Dighupara basins yields mainly Cambrian-Ordovician (524-474 Ma) ages. In contrast, the Barakar Formation from the Jharia Basin shows more scattered ages (1885-460 Ma) with multiple significant peaks in the Neoproterozoic and the dominant peak at Cambro-Ordovician, while the Talchir Formation has a contrasting Neoproterozoic age population of 895 Ma to 810 Ma, with the principal mode at 860 Ma indicating changes in the source terranes. Taken together, data suggest that the Eastern Ghats Mobile Belt of the Kuunga orogenic belt is the primary contributor to the younger early Permian strata, along with minor contributions from the Prydz Bay Belt and northern Prince Charles Mountains of the East Antarctic Craton. On the contrary, the Chotanagpur Granite Gneiss Complex of the Indian Craton and the Prydz Bay Belt on the East Antarctic Craton was the main source of the older Permo-Carboniferous Talchir Formation.

The tectonic configuration and evolution of southern Appalachian orogenic episodes are constrained, to a large extent, by knowledge of the timing and environment of pre- or syn-orogenic sediment deposition along with their subsequent thermal and deformational histories. The orogenic events that gave rise to regional folding and Barrovian metamorphism of the Western Blue Ridge (WBR) terrane are a classic example. We present new U-Pb and 40Ar-39Ar results bearing on the timing of magmatism and subsequent metamorphism of the Marble Hill Hornblende Schist (MHHS) of the WBR (southeastern USA). The schist is a key marker unit in the Murphy synclinorium, a central structure in the tectonic configuration of the WBR. The MMHS originated as basaltic volcanics overlying the Murphy Marble at the base of the Mineral Bluff Group. Ocean island basalt geochemistry and a zircon U-Pb age of ca. 437 Ma for the MHHS is consistent with plume-related volcanism in a Silurian remnant ocean basin of the Iapetus Ocean. Hornblende 40Ar-39Ar and rutile U-Pb ages for the MHHS are ca. 380−340 Ma and ca. 320 Ma, respectively, and record subsequent amphibolite facies metamorphism during Devonian Acadian orogeny and the deformation through regional folding and thrusting during the Carboniferous Alleghanian orogeny. The new age constraints for the MHHS require that “isograds” mapped in the region of the Murphy Belt are actually polymetamorphic mineral distributions that developed at least in part through post-Silurian tectonic events.

A narrow (<1 m) fine- to medium-grained lamprophyre dyke intruded Neoproterozoic monzodiorite on the eastern side of Machias Seal Island in the northern Gulf of Maine about 19 km southwest of the island of Grand Manan, New Brunswick, Canada. The dyke is near-vertical and trends 015° to 025°, with two shoreline exposures about 680 m apart assumed to be the same dyke. The dyke is notably dark and dense, with a granular broken surface. Thin sections reveal abundant euhedral outlines of olivine pseudomorphs in a groundmass of small (<0.2 mm) grains of brown amphibole (kaersutite) and augite, with ocelli and interstitial patches of albite, calcite, and analcite. The olivine phenocrysts have been completely replaced by chlorite, whereas other ferromagnesian minerals remain unaltered. Accessory minerals include abundant needle-shaped apatite, magnetite, and cubic pyrite. Another small dyke on the western side of the island is less mafic, non-porphyritic, and consists of fine- to medium-grained plagioclase and clinopyroxene, which are highly altered; we interpret it to be unrelated to the lamprophyre dyke to the east. Whole-rock chemistry shows that the lamprophyre dyke is camptonite, similar to some Mesozoic lamprophyre dykes of Maritime Canada and New England, USA, of which the nearest example is about 110 km to the southwest in coastal Maine. Laser fusion 40Ar/39Ar analyses of single kaersutite crystals (n = 12) yield an age distribution with a single, well-defined mode of ca. 468 Ma; incremental heating analyses of small aliquots of crystals show evidence of some radiogenic 40Ar loss and yield a mean plateau age of 478.7 ± 1.9 Ma. This Early Ordovician age and the accompanying uncertainty are interpreted to represent the time of crystallization for the camptonite dyke. The camptonite of Machias Seal Island is older than other Paleozoic and Mesozoic mafic dykes in the region. It is interpreted to be the product of partial melting of a metasomatised mantle during the Penobscot orogeny.

Extensive fieldwork and supporting laboratory analyses by Murray and Busby in the Cerocahui-Guazapares region of the northern Sierra Madre Occidental silicic large igneous province have identified three Oligocene volcanic stratigraphic subdivisions that were erupted during distinct phases of the mid-Cenozoic ignimbrite flare-up in western North America. The ca. 27.5 Ma Parajes Group, an ~1-km-thick sequence of rhyodacitic welded ignimbrite sheets, represents medial outflow facies erupted outside of the study area from unidentified caldera sources during the Oligocene pulse of flare-up magmatism. The ca. 27.5–24.5 Ma Témoris Formation is composed of Southern Cordillera basaltic andesite (SCORBA) mafic-intermediate lavas and associated intrusions, alluvial deposits, and distal nonwelded silicic ignimbrites deposited in synvolcanic half-graben basins following the Oligocene ignimbrite pulse. The ca. 24.5–23 Ma Sierra Guazapares Group is a fissure-fed silicic ignimbrite and bimodal volcanic unit that was erupted during the initiation of the early Miocene pulse of the ignimbrite flare-up. Three new 40Ar/39Ar ages further refine the ages of deposition and extension in the Cerocahui-Guazapares region. The Chepe ignimbrite, the lowest stratigraphic unit of the Parajes Group, yields a late Eocene age of 34.89 ± 0.11 Ma. This age is older than the majority of the Parajes Group—a new date from the KM ignimbrite near the stratigraphic top of the Parajes Group yields an age of 27.62 ± 0.3 Ma, which corresponds well to the previous ca. 27.5 Ma zircon U-Pb geochronology ages. In the Cerocahui basin, upper Témoris Formation alluvial deposits are capped by a Sierra Guazapares Group basalt lava unit that yields an age of 23.99 ± 0.20 Ma. This basalt lava has only minor offset across the basin-bounding fault, and much of the Sierra Guazapares Group is relatively flat, suggesting that extension in the study region was active since at least ca. 27.5 Ma but was negligible after ca. 23 Ma. The timing of extension and volcanism in the Cerocahui-Guazapares region is older than in areas further west, supporting the wide-rift to narrow-rift evolution models proposed for the Sierra Madre Occidental and the Gulf of California divergent plate margin.

In northwestern Turkey, within the Central Sakarya Terrane, the S & ouml;& gbreve;& uuml;t Metamorphics is one of the Variscan tectonic units preserved as a pre-Jurassic basement assemblage of the Sakarya Composite Terrane (SCT). It consists of para- and orthogneisses that host bands, lenses, boudins, and tectonic slices of amphibolites, and are altogether intruded by the Sar & imath;cakaya Granitoid. The characterization and timing of the precursor magmatic events for the protoliths of this assemblage and the timing and conditions of metamorphism have been the topics of a long-lasting debate. Here we present new geochemical and geochronological data from the S & ouml;& gbreve;& uuml;t orthogneisses, amphibolites, and the crosscutting Sar & imath;cakaya Granitoid to provide further understanding of the geodynamic evolution of the SCT Basement and the larger-scale tectonic events that controlled its formation. Available geological and geochronological data indicate that the generation of the orthogneiss protoliths began in the Late Cambrian (ca. 485 Ma) and continued until the middle Silurian (ca. 430 Ma), accompanied by the subduction of the Iapetus Ocean and its progressive roll-back beneath northern Gondwana. The slab roll-back caused mantle-upwelling and Ordovician (ca. 465 Ma) continental rift-related mafic magmatism, eventually leading to the opening of the Rheic Ocean. Prior to the Variscan orogeny, a short-lived episode of middle Carboniferous (ca. 336-328 Ma) intra-oceanic arc magmatism occurred within the Rheic Ocean north of the SCT. Accretion of all this material and continuous subduction beneath the SCT led to a two-stage metamorphism during the late Carboniferous (ca. 326-324 Ma and ca. 318 Ma) and synchronous generation of granitic (ca. 327-324) and dioritic (ca. 319- 317 Ma) rocks of the Sar & imath;cakaya Granitoid during the Variscan orogeny, terminating the life cycle of the Rheic Ocean.

Western Anatolia (WA) hosts numerous I-and S-type plutons with variable compositions. This paper presents new comprehensive geology, petrography, bulk-rock and mineral chemistry, Sr-Nd isotope, zircon U-Pb, biotite and hornblende Ar/Ar and first apatite U-Th / He age data for seven plutons (Baklan, Evciler, Eybek, Ilica-S,amli, Kozak, Uludag, Yenice) in WA. Western Anatolia granitoid plutons (WAGPs) are mainly granodiorite, with minor monzonite, diorite, quartz monzonite and granite. The dominant minerals are K-feldspar (usually orthoclase and rarely microcline), plagioclase (albite, oligoclase), amphibole (hornblende), biotite, quartz, and accessory minerals are titanite, zircon, apatite, and opaques. The crystallization temperatures are relatively uniform (640-740 degrees C) but pressures range from those typical of the upper crust to those typical of the mid crust. The plutons are characterized by moderate to high SiO2 (58.1-73.7 wt%), K2O (2.1-6.9 wt%), and relatively low P2O5 (0.03-0.60 wt%) and most are I-type and metaluminous, except for the Uludag pluton that is S-type. They are enriched in large ion lithophile elements (LILEs e.g. U, Th, Rb, and K), light rare earth elements (LREEs) and Pb, and depleted in heavy rare earth elements (HREEs), high field strength elements (HFSE, e.g. Nb, Ti) and Sr, Ba and P relative to other LILEs. The plutons display small Eu anomalies (Eu/Eu*: 0.17-1.15) due to probaly suppression of feldspar fractionation/or partial melting of lower crust. They have relatively high initial Sr-87/Sr-86 = 0.705775-0.708357 and low initial Nd-143/Nd-144 = 0.512290-0.512581. The LA-ICP-MS Pb-206-U-238 age analyses of zircon in addition to Ar-40/Ar-39 dating of hornblendes and biotites suggest that the plutons were emplaced between 29.4 Ma and 19.8 Ma and cooled with a relatively high rate (similar to 363 C/Ma). These ages correlate with the collision between the Sakarya Zone and Anatolide-Tauride Platform in Rupelian to Burdi-galian time. The (U-Th)/He apatite ages range from 19.8 Ma to 7.6 Ma, for the WA granitoid plutons and indicates that the crustal terrains hosting the seven plutons had complex exhumation histories within their arc environment. Consequently, these ages constrain the crystallization, cooling and exumation times of the Oligocene to Miocene plutons in a post-collisional setting on the northern margin of Gondwana.

The Galatean Volcanic Province (GVP) lies within the Sakarya tectonic belt in the northwest of central Anatolia, Turkey, south of the North Anatolian Fault, cropping out over 12,000 km(2). It consists of Early Miocene intermediate to acid lavas, pyroclastic rocks, volcaniclastic deposits, and Late Miocene OIB-like basalts. Our study is based on new bulk geochemistry, SrNd isotopes, and Ar-40/Ar-39 dating of the volcanic rocks from the south-western part of GVP, but integrates all the available previous data to understand how magmas evolved in the post-collisional geodynamic condition in the GVP. The initial eruptions were rhyolitic, as domes or pyroclastic deposits (Group 1) and basaltic lavas (Group 2) during the Early Miocene. Group 2 is also represented by large volumes of basaltic-andesitic, trachyandesite-dacite/trachydacite lava flows, small intrusions, and pyroclastic deposits as generated between 21 and 14 Ma. Relatively low Sr-87/Sr-86 ratios (0.705-0.706) of the early rhyolites (Group 1), similar to all the rocks from Group (2), suggest the generation of hybrid melts with variable contributions of mantle-derived and crustal material. The volcanic activity ends with OIB-like basalts (11-7 Ma) showing the lowest Sr-87/Sr-86 (similar to 0.703) suggesting an asthenospheric origin. The geodynamic model, based on post-Cyprian slab rollback, results in long-term (22-13 Ma) post-collisional delamination/drip processes that support magmatism. This magmatism is generated in the lithospheric mantle, with the formation of basaltic melts and acid hybrid melts showing variable contributions of mantle-derived and crustal materials in a complex trans-crustal magma plumbing system. Complex mixing of various intra-crustal magmas and fractional crystallization processes generated a huge volume of volcanic rocks. The Late Miocene small-volume OIB basalts were asthenospheric melts that during the late stage recorded localized decompression melting processes.

ABSTRACTWetumpka impact structure is a Late Cretaceous, marine‐target impact crater of about 5 km diameter. The apparent crater rim is mostly made of crystalline local basement, and the apparent crater floor consists of a mixed sediments of target lithology. These sediments are the provenance of the crater‐filling impactite sands, overlying trans‐crater slide unit, and the capping polymict impact breccia deposit, often referred to by previous workers as “central polymict breccia.” The unit has been known to contain elongated mega‐clasts of up to tens of meters in size. This study attempted to understand the mode of emplacement of this polymict breccia, which occurs in some places on the apparent crater floor and resembles a polymict proximal ejecta deposit. This work also reports the first documentation of rare, potential impact spherules in the polymict impact breccia, interpreted to be a part of distal ejecta. The presence of large, decimeter‐sized clasts in the breccia can be best explained by the movement of overturned rim flap forming part of proximal ejecta from the crater rim to the apparent crater floor during early modification stage of impact cratering. Our work highlights the bimodal clast size distribution of the polymict breccia, and so we propose that the term “mega‐clast‐bearing impact breccia” be used for this unit. We attribute a generally steep orientation of the decameter sized clasts to primary imbrication during emplacement. The emplacement of this breccia is interpreted as associated with the ejecta emplacement process that occurred before the return of marine resurge.

CM chondrites are samples from primitive water-rich asteroids that formed early in the solar system; many record evidence for silicate rock-liquid water interaction. Many CM chondrites also exhibit well-developed fine-grained rims (FGRs) that surround major components, including chondrules and refractory inclusions. Previous studies have shown that Aguas Zarcas, a CM2 chondrite fall recovered in 2019, is a breccia consisting of several lithologies. Here, we present a study of Aguas Zarcas using optical microscopy, scanning electron microscopy, and electron probe microanalysis, focusing on brecciation and aqueous alteration on the parent body. We observed two lithologies within our sample, separated by a distinct textural and chemical boundary. The first lithology has a higher chondrule abundance ("chondrule-rich") and significantly larger FGRs compared to the second lithology ("chondrule-poor"), even for similarly sized chondrules. We observed clear compositional differences between the two lithologies and more multilayered FGRs in the chondrule-rich lithology. We determined that the chondrule-rich lithology is less altered (petrologic type 2.7-2.8) and displays larger FGRs to chondrule ratios compared to the more altered chondrule-poor lithology (petrologic type 2.5-2.6). These observations are contrary to previous models that predict aqueous alteration as a cause of FGR formation in the parent body. Our observed differences in Mg and Fe distribution in the lithology matrices alongside variable FGR thickness suggest distinct formation environments. We propose that the Aguas Zarcas parent body was subjected to several minor and major brecciation events that mixed different materials with variable degrees of aqueous alteration together, in agreement with previous studies.

Abstract Basaltic lavas from Harrat Uwayrid, Saudi Arabia, record the evolving magmatic and tectonic context of the Arabian Peninsula from at least the mid‐Miocene to the present day. New 40Ar/39Ar ages spanning from the mid to late Miocene reveal that mid‐Miocene mafic volcanism formed a large, subalkaline volcanic plateau parallel to Red Sea rifts. Subsequent volumetrically subordinate late Miocene‐Quaternary alkaline volcanism erupted monogenetic cinder cones roughly orthogonal to the earlier volcanic field. The source region for all samples was affected by both fluid and silicate metasomatism; inferred mantle mineral assemblages include amphibole for mid‐Miocene lavas and phlogopite for late Miocene‐Quaternary samples. Calculated melting depths become shallower with time across the Miocene volcanic episode (∼20–15 Ma) but become deeper in the late Miocene to Quaternary (∼10–0 Ma), indicating melting pressures and temperatures significantly higher than those recorded in Miocene lavas despite progressive lithospheric thinning. We offer a two‐stage model for the formation of Harrat Uwayrid: (a) Early‐ and mid‐Miocene rifting associated with the Red Sea opening facilitated adiabatic melting of uppermost mantle lithosphere to form the early volcanic plateau and (b) Plate motion changes in the mid‐ and late‐Miocene initiated the Dead Sea Fault and destabilized a dense pyroxenitic lower lithosphere leading to foundering or lithospheric drip beneath Harrat Uwayrid that allowed deep lithospheric melting and formed the young volatile‐rich eruptives.

The record of Permian–Triassic evolution in eastern North America indicates an important change in the tectonic regime from compression to extension as eastern Laurentia transitioned from the Alleghanian orogeny to continental rifting associated with the breakup of Pangea. The temporal pace (e.g., gradual vs. episodic, diachronous vs. synchronous), the accommodating structures, and the influential processes that characterized this transition provide critical insights into the late Paleozoic evolution of Laurentia and rifted continental margins in general. Connections between the formation of the South Georgia basin and regional cooling of the southernmost Appalachian crystalline rocks, along with the distribution of normal faults and discontinuities in metamorphic grade, indicate extensional collapse of the Alleghanian orogen along an extensive detachment system that was active from ca. 295 to 240 Ma. The 40Ar/39Ar cooling ages of biotites from low-angle normal shear zones cutting migmatitic gneisses of the southernmost Appalachians are interpreted to document extensional faulting ca. 280 Ma and to provide a snapshot of the prolonged orogenic collapse. The timing, orientation of structures, extent of reactivation, and character of late Alleghanian extension in the central and northern Appalachians provide an orogen-scale framework for this tectonic transition. This contribution focuses on correlations between the beginning of orogenic collapse and the initiation of continental rifting along with the tectonic processes that transformed eastern North America from a convergent to divergent plate boundary following the Alleghanian orogeny.

The Qilian orogen at the northeastern margin of the Tibetan Plateau records a complete history from continental breakup to oceanic basin evolution and back to continental collision during the Neoproterozoic to Paleozoic interval. The Qilian Ocean, as a branch of the Proto-Tethys Ocean, was connected to the Iapetus Ocean but is lesser known than its two more famous counterparts. This paper presents zircon U-Pb and biotite-muscovite 40Ar/39Ar isotope data from the Yemananshan Complex in the western Central Qilian belt to assess the age and provenance of late Mesoproterozoic-early Paleozoic successions in the Qilian orogen. The results indicate the Central Qilian belt is characterized by Late Mesoproterozoic-early Neoproterozoic orogenesis with magmatism from 1200 Ma to 900 Ma and clastic metasedimentary rocks deposited between 1050 and 920 Ma. Statistical analysis of detrital zircon data and auxiliary temporal information from the Qilian orogen affirms the notion that sedimentation occurred in four distinct megasequences: (1) 1950-1850 Ma, the oldest known strata, (2) 1250-920 Ma, during Grenvillian-Sveconorwegian orogenesis, (3) 840-760 Ma, the disintegration of Rodinia, and (4) 650-550 Ma, which is typical of the orogenic belts that sutured the Gondwana continental blocks. Metamorphic zircon and 40Ar/39Ar isotope data indicate early Paleozoic metamorphism at ca. 475 Ma, which overlaps with magmatic activity associated with arc-back-arc basin development in the Central Qilian belt, which was associated with subduction of the Proto-Tethys in eastern Gondwana.