Search

The Baker terrane, exposed in the Blue Mountains province of northeastern Oregon, is a long-lived, ancient (late Paleozoic-early Mesozoic) accretionary complex with an associated forearc. This composite terrane lies between the partially coeval Wallowa and Olds Ferry island-arc terranes. The northern margin of the Baker terrane is a broad zone (>25 km wide) of fault-bounded, imbricated slabs and slices of metaigneous and meta-sedimentary rocks faulted into chert-argillite melange of the Elkhorn Ridge Argillite. Metaplutonic rocks within tectonic units in this zone crystallized between 231 and 226 Ma and have low initial [sup.87]Sr/[sup.86]Sr ratios (0.7033-0.7034) and positive initial ENd values (+7.7 to +8.5). In contrast, siliceous argillites from the chert-argillite melange have initial [sup.87]Sr/[sup.86]Sr values ranging from 0.7073 to 0.7094 and initial [[epsilon].sub.Nd] values between -4.7 and -7.8. We interpret this broad, imbricate fault zone as a fundamental tectonic boundary that separates the distal, Wallowa island-arc terrane from the Baker accretionary-complex terrane. We propose that this terrane boundary is an example of a broad zone of imbrication made up of slabs and slices of arc crust tectonically mixed within an accretionary complex, providing an on-land, ancient analog to the actualistic arc-arc collisional zone developed along the margins of the Molucca Sea of the central equatorial Indo-Pacific region. doi: 10.1130/B26493.1

Midcrustal (25-30 km) emplacement of the dioritie Sausfjellet pluton into rocks of the Helgeland Nappe Complex, central Norway, occurred in two stages. Stage 1 consists of two-pyroxene hornblende gabbro and diorite. Stage 2 is asymmetrically zoned, with a modally layered central zone of diorite and anorthosite and a western/annular zone of quartz-bearing monzodioritic rocks. Igneous layering was locally attenuated, folded, and boudinaged in the hyper-solidus state. The magmatic foliation trajectory pattern in the pluton defines a shallowly southwest-plunging synform that crosscuts compositional zones. Mineral lineations plunge shallowly to moderately to the southwest. The pluton intruded a major lithologic boundary within the nappe; migmatitic pelitic gneisses are the main host rocks to the western part of the pluton, whereas the eastern and central parts arc hosted predominantly by metacarbonate rocks. Calc-silicate, marble, quartzo-feldspathic, and dioritic xenoliths (up to 200 m in length) are present throughout the pluton; they are most common in Stage 1 and the central zone of Stage 2. Metapelitic xenoliths are conspicuously absent. Ductile flow during emplacement produced an ~1-km-wide structural aureole in which host-rock structures were deflected into subparallelism with the steeply inward-dipping margin of the pluton. Tight antiforms developed along the northeastern, southeastern, and southwestern margins. Amphibolite-grade shear zones in the host rocks preserve pluton-side-up kinematic indicators. In addition to the abundance of dioritic, calc-silicate, and quartzo-feldspathic gneiss xenoliths and geochemical evidence for assimilation in the western/annular zone, regional discordance of the pluton-host-rock contacts indicates that stoping was also an important emplacement process at midcrustal depths. Following magma emplacement, foundering of the central portion of the chamber combined with possible ca. 445 Ma regional contraction produced the map-scale synform defined by magmatic foliations and igneous layers. This study demonstrates that stoping and assimilation may occur simultaneously with host-rock ductile flow during magma chamber evolution at midcrustal levels and offers an explanation of why xenolith preservation may be compositionally dependent. Keywords: pluton emplacement, stoping, layered intrusions, Caledonides, Bindal Batholith.

Magmatism, contractional deformation, and extension associated with the exhumation of high-pressure rocks in the Scandinavian Caledonides are commonly attributed to the Silurian-Devonian Scandian orogeny, in which eastward thrusting of allochthonous terranes over Baltica was followed by extensional collapse and exhumation. New fieldwork and U-Pb geochronology coupled with recent pressure-temperature estimates within the highest thrust sequence of the Caledonian orogen indicate that an earlier phase of west-directed contractional deformation was punctuated by migmatite-producing events and voluminous magmatism ca. 477-466 Ma and ca. 447 Ma, followed by exhumation in the Late Ordovician. Al-in-hornblende and GASP thermobarometry indicate that emplacement of a suite of 448-445 Ma plutons caused partial migmatization at pressures of 700-800 MPa. Subsequent isothermal exhumation to pressures of 400 MPa occurred while the host rocks were still partially molten. Rates of exhumation may have ranged from 2 to 11 mm * [yr.sup.-1] or greater. These data provide evidence for a previously unrecognized phase of exhumation in the Caledonides and for aerially extensive west-vergent deformation. Deformation and magmatism associated with these events may be related to Taconic-age orogenesis near Laurentia, where the highest nappe sequences of the Scandinavian Caledonides probably resided during early Paleozoic time. Keywords: exhumation, Taconian orogeny, Caledonides, migmatite, SHRIMP U-Pb.

We present microbeam major- and trace-element data from 14 monzodiorites collected from the Malaspina Pluton (Fiordland, New Zealand) with the goal of evaluating processes involved in the production of andesites in lower arc crust. We focus on relict igneous assemblages consisting of plagioclase and amphibole with lesser amounts of clinopyroxene, orthopyroxene, biotite and quartz. These relict igneous assemblages are heterogeneously preserved in the lower crust within sheeted intrusions that display hypersolidus fabrics defined by alignment of unstrained plagioclase and amphibole. Trace-element data from relict igneous amphiboles in these rocks reveal two distinct groups: one relatively enriched in high field strength element concentrations and one relatively depleted. The enriched amphibole group has Zr values in the range of ∼25–110 ppm, Nb values of ∼5–32 ppm, and Th values up to 2·4 ppm. The depleted group, in contrast, shows Zr values <35 ppm and Nb values <0·25 ppm, and Th is generally below the level of detection. Amphibole crystallization temperatures calculated from major elements range from ∼960 to 830 °C for all samples in the pluton; however, we do not observe significant differences in the range of crystallization temperatures between enriched (∼960–840 °C) and depleted groups (∼940–830 °C). Bulk-rock Sr and Nd isotopes are also remarkably homogeneous and show no apparent difference between enriched (εNdi = 0·1 to –0·1; 87Sr/86Sri = 0·70420–0·70413) and depleted groups (εNdi = 0·3 to –0·4; 87Sr/86Sri = 0·70424–0·70411). Calculated amphibole-equilibrium melt compositions using chemometric equations indicate that melts were highly fractionated (molar Mg# <50), andesitic to dacitic in composition, and were much more evolved than bulk lower continental crust or primitive basalts and andesites predicted to have formed from hydrous melting of mantle-wedge peridotite beneath an arc. We suggest that melts originated from a common, isotopically homogeneous source beneath the Malaspina Pluton, and differences between enriched and depleted trace-element groups reflect varying contributions from subducted sediment-derived melt and sediment-derived fluid, respectively. Our data demonstrate that andesites and dacites were the dominant melts that intruded the lower crust, and their compositions mirror middle and upper bulk-continental crust estimates. Continental crust-like geochemical signatures were acquired in the source region from interaction between hydrous mantle-wedge melts and recycled subducted sediment rather than assimilation and/or remelting of pre-existing lower continental crust. [ABSTRACT FROM AUTHOR]

Mantle rocks produce mafic precursor magmas upon melting, which preceded the early cretaceous magmatism in the Klamath mountains. Under optimum tectonic and thermal conditions, mafic crystal rocks change to form low- and high-Al tonalite and trondhjemite magmas. Tonalite-trondhjemite plutons characterize the early cretaceous magmatism in the Klamath mountains. An external heat source derived from basaltic melts is necessary for the partial melting of mantle rocks leading to the origin of tonalite-trondhjemite magmas.

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]