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The Late Jurassic English Peak plutonic complex was emplaced in an upper crustal retro-arc setting in the central Klamath Mountains province, northern California. Emplacement of the main, central pluton was preceded by intrusion of two satellite bodies: the Uncles Creek pluton crystallized from H2O-rich quartz dioritic magma with hornblende as the liquidus mafic phase; in contrast, the Heiney Bar pluton is a c. 25km diameter body zoned from gabbro to granodiorite. Al-in-hornblende barometry from these two plutons indicates a stage of magma storage at c. 600- 500 MPa. The central English Peak pluton is a c. 15km diameter body composed of early and late stages. Early stage rocks range from gabbro to tonalite, with variable proportions of augite, orthopyroxene, hornblende and biotite. The early stage lacks discernible zoning and rock types vary at the outcrop scale. This diversity is reflected in bulk-rock compositions, which do not form a compositional array. The late-stage intrusion consists of three concentric units that are zoned from outer, more mafic rocks (quartz diorite, tonalite, quartz monzodiorite) to inner, compositionally evolved rocks (granodiorite and granite). Late-stage samples plot in smooth, typically linear arrays for most major and trace elements. Al-in-hornblende pressures indicate that late-stage hornblende cores grew in a reservoir at c. 400MPa and that rims grew at the level of final emplacement (c. 250 MPa). The mid-crustal reservoir was the site of late-stage magma evolution, including episodic magma mixing. Oxygen and Sr isotopes indicate initial evolution of English Peak pluton magmas in a deep crustal region of mixing, assimilation, storage, and homogenization (MASH zone), where they were contaminated by metasedimentary rocks. Thus, the English Peak pluton represents a crustalscale system, with mantle-derived magmas that differentiated near the Moho, storage and crystallization of satellite-pluton magmas in the middle crust (c. 600-500 MPa), development of a large, episodically recharged, magma chamber in the upper middle crust (c. 400 MPa) and final emplacement in the upper crust. [ABSTRACT FROM AUTHOR]

The granitoid suites encountered by drilling in the northern South China Sea (SCS) remain important for understanding the evolution of the late Mesozoic Southeast Asian continental margin. They comprise a range of rock types including diorite, tonalite, granodiorite, monzogranite and syenogranite with SiO2spanning 56.4–76.8%. Newly acquired secondary ion mass spectrometry (SIMS) U–Pb ages of samples from 14 boreholes indicate two key magmatic episodes: Late Jurassic (161.6–148.2 Ma) and Early Cretaceous (136.5–101.7 Ma). Jurassic magmatism probably began in late Middle Jurassic time, documented by the dates of inherited zircons. The granitoids are dominated by metaluminous to weakly peraluminous I-type granites, are transitional between magnesian and ferroan, and encompass calc-alkaline, high-K calc-alkaline, and shoshonitic series. The geochemical signatures suggest that these granitoids were mostly generated in a normal continental arc environment. Notable features of the I-type samples are well-defined negative Nb–Ta–Ti anomalies typical of arc-related magmas. Taken together, the late Mesozoic arc granites of the SCS, the accretionary wedge of the Palawan terrane to the southeast, and the zone of lithospheric extension north of the SCS throughout Southeast China, define a southeast-to-northwest trench-arc-backarc architecture for the late Mesozoic Southeast Asian continental margin whose geodynamic setting is related to subduction of the Palaeo-Pacific slab beneath the Asian continent. Two key subduction episodes are recognized, one in Late Jurassic and the other in Early Cretaceous time. [ABSTRACT FROM PUBLISHER]

The structurally lowest parts of the Upper Nappe of the Helgeland Nappe Complex consist primarily of migmatitic quartzofeldspathic gneiss with intercalated calc-silicate gneiss. Detrital zircons in the gneiss indicate an Ordovician depositional age. Migmatization via biotite dehydration reactions occurred at ∼480 Ma at pressures of ∼700 MPa; some migmatites were mobilized into cross-cutting diatexites. Diverse gabbroic to granitic plutons and dikes of the Bindal Batholith were emplaced from 448-424 Ma. They range from calc-alkaline to alkalic and most have initial 87Sr/86Sr <0.707 and ϵNd from 0 to -4. Pressures of emplacement were ca. 300 MPa. Shear zones with reverse shear sense cut 437 Ma intrusions, suggesting that contraction accompanied magmatism. [ABSTRACT FROM AUTHOR]

Abstract: The inferred subduction affinity of the ∼1780-Ma Green Mountain arc, a dominantly bimodal igneous terrane (together with immature marine and volcaniclastic sedimentary rocks) accreted to the southern margin of the Wyoming province, is integral to arc-accretion models of the Paleoproterozoic growth of southern Laurentia. Conversely, the dominantly bimodal nature of many putative arc-related igneous suites throughout southern Laurentia, including the Green Mountain arc, has also been used to support models of growth by extension of pre-existing crust. We report new geochemical and isotopic data from ∼1780-Ma gabbroic and granodioritic to tonalitic rocks of the Big Creek Gneiss, interpreted as consanguineous with previously studied metavolcanic rocks of the Green Mountain Formation. The ∼1780-Ma Big Creek Gneiss mafic rocks show clear geochemical signatures of a subduction origin and provide no supporting evidence for extensional tectonism. The ∼1780-Ma Big Creek Gneiss felsic rocks are attributed to partial melting of mafic and/or mixed lower-crustal material. The bimodal nature of the suite results from the combination of arc basalts and felsic crustal melts. The lack of andesite is consistent with the observed tholeiitic differentiation trend of the mafic magmas. The lower ɛ Nd(1780Ma) values for the felsic rocks vs. the mafic rocks suggest that the unexposed lower crust of the arc may be older than the arc and that Trans-Hudson- or Penokean-aged rocks possibly form the substratum of the arc. Our results reinforce previous interpretations that arc-related magmatism played a key role in the Paleoproterozoic crustal growth of southern Laurentia, but also support the possibility of unexposed older crust as basement to the arcs. [Copyright &y& Elsevier]

The Helgeland Nappe Complex consists of a sequence of imbricated east-dipping nappes that record a history of Neoproterozoic-Ordovician, sedimentary, metamorphic, and magmatic events. A combination of U-Pb dating of zircon and titanite by laser-ablation-inductively coupled plasma-mass spectrometry plus chemostratigraphic data on marbles places tight constraints on the sedimentary, tectonic, and thermal events of the complex. Strontium and carbon isotope data have identified Neoproterozoic marbles in the Lower Nappe, the Horta nappe, and Scandian-aged infolds in the Vikna region. The environment of deposition of these rocks was a continental shelf, presumably of Laurentia. Detrital zircon ages from the Lower Nappe are nearly identical to those of Dalradian sedimentary rocks in Scotland. Cambrian rifting caused development of one or more ophiolite-floored basins, into which thick sequences of Early Ordovician clastic and carbonate sediments were deposited. On the basis of ages of the youngest zircons, deposition ended after ca. 481 Ma. These basin units are now seen as the Skei Group, Sauren-Torghatten Nappe, and Middle Nappe, as well as the stratigraphically highest part of the Horta nappe and possibly of the Upper Nappe. The provenance of these sediments was partly from the Lower Nappe, on the basis of detrital zircon age populations in metasandstones and cobbles from proximal conglomerates. However, the source of Cambrian-Ordovician zircons in all of the Early Ordovician basins is enigmatic. Crustal anatexis of the Lower and Upper Nappes occurred ca. 480 Ma, followed by imbrication of the entire nappe sequence. By ca. 478 Ma, the Horta nappe was overturned and was at the structural base of the nappe sequence, where it underwent migmatization and was the source of S-type magmas. Diverse magmatic activity followed ca. 465 Ma, 450–445 Ma, and 439–424 Ma. Several plutons in the youngest age range contain inherited 460–450 Ma zircons. These zircons are interpreted to reflect a deep crustal zone in which mafic magmas caused melting, mixing, and hybridization from 460 to 450 Ma. Magmatic reheating of this zone, possibly associated with crustal thickening, resulted in voluminous, predominantly tonalitic magmatism from 439 to 424 Ma. [ABSTRACT FROM AUTHOR]

Evaluates the orodivician magmatism in the Caledonides of central Norway. Provision of evidence for the unrecognized phase of exhumation in the Caledonides; Relation between deformation and Taconic-age orogenesis near Laurentia; Indication of the Al-in-hornblende and GASP thermobarometry.

Two distinct basaltic suites characterize ∼1165- to 1120-Ma magmatism in west Texas. The ∼1163 Ma Pecos mafic intrusive complex consists of layered mafic and ultramafic rocks. Megacyclic units record repeated influx and mixing of tholeiitic magmas to yield a predominantly noritic body. Basaltic magmatism also accompanied emplacement of the 1120 ± 35-Ma Red Bluff granitic suite in the Franklin Mountains. In terms of their major elements, these basaltic rocks are transitional between tholeiitic and alkaline, but their low concentrations of Nb suggest a subduction-related origin. Except for their low Nb contents, the Franklin Mountains basaltic rocks are similar to coeval basaltic intrusive and volcanic rocks in the southwestern U.S.A. Of the numerous proposed tectonic settings, an extensional environment is preferred. Possible extensional associations include splays of the Midcontinent Rift, rifting caused by collision of a continental fragment during the Grenville orogeny, and extension related to collapse of over-thickened crust during the Grenville event. The low Nb contents of basalts associated with Franklin Mountains basaltic rocks are best explained as an artifact of older subduction, which modified the mantle source of the basalts hundreds of millions of years prior to their emplacement. [ABSTRACT FROM AUTHOR]

Although much evidence points to a Late Mesozoic large silicic igneous province in SE South China (SESC), diverse Andean-type and non-Andean-type models have been proposed to explain its origins. New age, isotope, and trace element data on zircon from wells in East China Sea (ECS) and offshore-onshore tectonomagmatic comparisons allow reassessment of the geodynamic interactions (ca. 200–86 Ma) between the Cathaysia-based arc and intraplate activity from a large mantle wedge framework. In region, remnants of the West ECS arc, Ryukyu-Taiwan mélange, and SESC intraplate define a trench–arc–intraplate architecture. Changes in arc magmatic states (flare-ups and lulls; high U/Yb, low T , variable εHf (t)) permit identification of a cycle of arc initiation, thickening, decline, arc-root removal, and arc migration, highly correlating with changes of Izanagi subduction beneath East Asia. Voluminous intraplate silicic magmatism operated in response to the slab stagnation dynamics: slab dehydration, lithospheric delamination, fluid-fluxed crustal anataxis, and crustal–mantle melt hybridization in crustal hot zones. Such geodynamic scenarios, unlike an Andean-type continental margin, instead result in the construction of a Late Mesozoic Cathaysian-type active continental margin. • A model of non-Andean-type Cathaysian continental margin is proposed based on offshore-onshore tectonomagmatic correlations. • The model reveals a link between the arc, intraplate, and slab subduction systems from the large mantle wedge perspective. • Drilled data confirm the western East China Sea and Cathaysia domains sharing a unified Paleoproterozoic Cathaysian basement. [ABSTRACT FROM AUTHOR]

Abstract: Large Grenville-age plutons are known to be present in the 1340–1370 Ma basement in Texas and eastern New Mexico, but their distribution and compositional ranges are poorly known. New U–Pb zircon dates (SHRIMP-RG, Stanford University) on subsurface samples have identified widespread presence of compositionally diverse Grenville-age (1070–1110 Ma) plutonic rocks. In the Texas Panhandle, core from a >175 m thick, sill-like gabbroic intrusion yielded a date of 1081±8.3 Ma. In situ differentiation of this tholeiitic magma led to a ∼7-fold increase in incompatible element concentrations as REE patterns remained essentially flat. Trace element data suggest an E-MORB-like source. To the south, alkali–feldspar granite with A-type affinities was recovered from drill core from the Abilene gravity minimum near Albany, Texas. It yielded a date of 1078±23 Ma; similar to undeformed (post-orogenic) granites in the Llano uplift. Further west, an anorthosite xenolith from the Eocene Three Sisters intrusion in El Paso, Texas yielded a date of 1068±30 Ma which is within error of the 1110±19 Ma age determined for the main stage of the nearby A-type Red Bluff granitic suite. Xenoliths from Potrillo maar volcano in the center of the southern Rio Grande rift include monzonitic xenoliths with granulitic texture which yielded dates of ∼1072 Ma. In contrast, Potrillo maar xenoliths with igneous textures yielded U–Pb dates of ∼27 Ma. Development of the granulitic texture is interpreted to reflect granulite facies metamorphism during Tertiary time (26–27 Ma) on the basis of age zoning and U concentrations in the zircon crystals. Our results show that Grenville-age magmatism in the Texas and New Mexico subsurface was widespread and was coeval with syn- and post-deformation granites in the Llano uplift of central Texas. The compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were “A-type”. These magmatic affinities are thought to be associated with crustal extension rather than with subduction. Moreover, the presence of anorthosite xenoliths in the El Paso-Kilbourne Hole region, the broadly tholeiitic character of mafic rocks, and the A-type nature of the granitic rocks are broadly similar to AMCG (Anorthosite–Mangerite–Charnockite–Granite) suites present in the Grenville province of eastern Laurentia. [Copyright &y& Elsevier]

An early to mid-Mesozoic record of sedimentation, magmatism, and metamorphism is well developed in the Blue Mountains Province of northeast Oregon. Detailed studies both north and south of the Blue Mountains Province (e.g., terranes of the Inter montane belt, Klamath Mountains, and western Sierra Nevada) have documented a complex Middle to Late Jurassic orogenic evolution. However, the timing of magmatic, metamorphic, and deformational events in the Blue Mountains, and the significance of these events in relationship to other terranes in the western North American Cordillera remain poorly understood. In this study, we investigate the structural, magmatic, and meta morphic histories of brittle to semibrittle deformation zones that indicate widespread Late Jurassic orogenesis in the Blue Mountains Province. Folding and faulting associated with contractional deformation are primarily localized along terrane boundaries (e.g., Baker-Wallowa and Baker--Izee--Olds Ferry boundaries) and within the composite Baker oceanic mélange terrane (e.g., Bourne-Greenhorn subterrane boundary). These brittle to semibrittle deformation zones are broadly characterized by the development of E-W--oriented slaty to spaced cleavage in fine-grained metasedimentary rocks of the Baker terrane (e.g., Elkhorn Ridge Argillite), approximately N-S--bivergent folding, and N- and S-dipping reverse and thrust faulting on opposite flanks of the Baker terrane. Similarly oriented contractional features are also present in late Middle Triassic to early Late Jurassic (i.e., Oxfordian Stage, ca. 159 Ma) sedimentary rocks of the John Day and Huntington areas of northeast Oregon. Radiometric age constraints from youngest detrital zircons in deformed sedimentary rocks and crystallization ages of postkinematic plutons, which intrude the deformation zones, limit deformation to between ca. 159 and ca. 154 Ma. We suggest that the widespread, approximately N-S--directed contractional features in the Blue Mountains Province record a short-lived, intense early Late Jurassic deformational event and preserve an example of upper-crustal strain localization associated with terminal arc-arc collision between the Olds Ferry and Wallowa island-arc terranes. The age interval of deformation in the Blue Mountains Province is younger than Middle Jurassic deformation in the Canadian Cordillera and Klamath Mountains (Siskiyou orogeny) and predates classic Nevadan orogenesis. [ABSTRACT FROM AUTHOR]