Geochemical characteristics and conditions of formation of the Chah-Bazargan peraluminous granitic patches, ShahrBabak, Iran

Abstract Xenoliths of garnet-biotite-kyanite schist from the Qori metamorphic complex (southern part of the Sanandaj-Sirjan zone, northeast Neyriz, Zagros orogen in Iran) in the 173.0±1.6 Ma Chah-Bazargan leuco-quartz diorite intrusion were studied. This intrusion caused these schist xenoliths to be metamorphosed to the pyroxene hornfels facies (approximately 4.5±1.0 kbar and 760±35 °C), converting them to diatexite migmatite as a result of partial melting of the xenoliths. These melts are granites in composition. Melt volumes of 20 to 30 vol. % were calculated for small patches of the peraluminous granites. It is possible that anatectic melting affected only the leucosome, such that melting was more than 20 to 30 vol. %. It is possible that a large amount of melt was not extracted due to balanced in situ crystallization, the adhesion force between melt and crystal (restite), and high viscosity of the leucosome. The Chah-Bazargan peraluminous granites are depleted in trace elements such as REEs, HFSE (Ti, Zr, Ta, Nb, Th, U, Hf, Y), Ba, Pb, and Sr. These elements are largely insensitive to source enrichment, but sensitive to the amounts of main and accessory minerals. These elements were hosted by minerals such as garnet, biotite, muscovite, K-feldspar, plagioclase, ilmenite, apatite, monazite, and zircon in the source (diatexitic migmatitic xenoliths).

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Partial melting of oceanic sediments in subduction zones and its contribution to the petrogenesis of peraluminous granites in the Chinese Altai

Geological Magazine ◽

10.1017/s0016756817001029 ◽

2018 ◽

Vol 156 (4) ◽

pp. 585-604 ◽

Cited By ~ 1

Author(s):

QUN LUO ◽

CHEN ZHANG ◽

SHU JIANG ◽

LUOFU LIU ◽

DONGDONG LIU ◽

...

Keyword(s):

Partial Melting ◽

Subduction Zones ◽

Peraluminous Granite ◽

Isotopic System ◽

Central Asian ◽

Oceanic Sediments ◽

Peraluminous Granites ◽

Isotopic Analyses ◽

Geodynamic Processes

AbstractLate Carboniferous magmatism in the Chinese Altai provides an important view of geodynamic processes active during crustal growth in the Central Asian Orogenic Belt (CAOB). In this study, five representative peraluminous granite plutons from the Chinese Altai were selected for systematic geochronological, geochemical and Sr–Nd–Hf isotopic analyses (Table 1). These granites were emplaced between 449 and 327 Ma in an active subduction zone, and have moderate to high SiO2 (66.54–76.13 wt%), moderate Na2O+K2O (6.27–7.66 wt%), and high Al2O3 contents (12.43–16.18 wt%). All granite samples in this study showed significant decoupling of the Nd and Hf isotope systems. Results show negative εNd(t) values (−3.3 to −0.9), and predominantly positive εHf(t) values (+0.24 to +8.01, n=57) except for a few negative εHf(t) values (−7.44 to −0.03, n=9), high Mg# values (28.69–53.33), high Nd/Hf ratios (4.26–43.57), and enrichment of large-ion lithophile elements (LILEs; e.g. Pb, Th, and U), suggesting that the granites were derived from the partial melting of oceanic sediments and the associated mantle wedge, with fractionation of plagioclase, K-feldspar and biotite. In situ zircon Hf isotopic analyses yield negative εHf(t) values from −30.6 to −13.7 for the zircon xenocrysts. The U–Pb ages and Hf isotopic ratios of these zircon xenocrysts were probably inherited from oceanic sediments. Zircon saturation temperatures suggest that these peraluminous granites were emplaced at 537–765°C. We propose that: (1) the Nd isotopic system more faithfully reflects the source of peraluminous magmas in the Chinese Altai than the Hf isotopic system, and (2) the oceanic sediment recycling was an important process during continental growth in the CAOB.

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In-situ reaction-replacement origin of hornblendites in the Early Cretaceous Laiyuan complex, North China Craton, and implications for its tectono-magmatic evolution

Journal of Petrology ◽

10.1093/petrology/egab030 ◽

2021 ◽

Author(s):

Jia Chang ◽

Andreas Audétat ◽

Jian-Wei Li

Keyword(s):

Partial Melting ◽

North China Craton ◽

Lithospheric Mantle ◽

Lower Crust ◽

North China ◽

Ultramafic Rocks ◽

Magmatic Evolution ◽

Mafic Magmas ◽

Felsic Rocks

Abstract Two suites of amphibole-rich mafic‒ultramafic rocks associated with the voluminous intermediate to felsic rocks in the Early Cretaceous Laiyuan intrusive-volcanic complex (North China Craton) are studied here by detailed petrography, mineral- and melt inclusion chemistry, and thermobarometry to demonstrate an in-situ reaction-replacement origin of the hornblendites. Moreover, a large set of compiled and newly obtained geochronological and whole-rock elemental and Sr-Nd isotopic data are used to constrain the tectono-magmatic evolution of the Laiyuan complex. Early mafic‒ultramafic rocks occur mainly as amphibole-rich mafic‒ultramafic intrusions situated at the edge of the Laiyuan complex. These intrusions comprise complex lithologies of olivine-, pyroxene- and phlogopite-bearing hornblendites and various types of gabbroic rocks, which largely formed by in-situ crystallization of hydrous mafic magmas that experienced gravitational settling of early-crystallized olivine and clinopyroxene at low pressures of 0.10‒0.20 GPa (∼4‒8 km crustal depth); the hornblendites formed in cumulate zones by cooling-driven crystallization of 55‒75 vol% hornblende, 10‒20 vol% orthopyroxene and 3‒10 vol% phlogopite at the expense of olivine and clinopyroxene. A later suite of mafic rocks occurs as mafic lamprophyre dikes throughout the Laiyuan complex. These dikes occasionally contain some pure hornblendite xenoliths, which formed by reaction-replacement of clinopyroxene at high pressures of up to 0.97‒1.25 GPa (∼37‒47 km crustal depth). Mass balance calculations suggest that the olivine-, pyroxene- and phlogopite-bearing hornblendites in the early mafic‒ultramafic intrusions formed almost without melt extraction, whereas the pure hornblendites brought up by lamprophyre dikes required extraction of ≥ 20‒30 wt% residual andesitic to dacitic melts. The latter suggests that fractionation of amphibole in the middle to lower crust through the formation of reaction-replacement hornblendites is a viable way to produce adakite-like magmas. New age constraints suggest that the early mafic-ultramafic intrusions formed during ∼132‒138 Ma, which overlaps with the timespan of ∼126‒145 Ma recorded by the much more voluminous intermediate to felsic rocks of the Laiyuan complex. By contrast, the late mafic and intermediate lamprophyre dikes were emplaced during ∼110‒125 Ma. Therefore, the voluminous early magmatism in the Laiyuan complex was likely triggered by the retreat of the flat-subducting Paleo-Pacific slab, whereas the minor later, mafic to intermediate magmas may have formed in response to further slab sinking-induced mantle thermal perturbations. Whole-rock geochemical data suggest that the early mafic magmas formed by partial melting of subduction-related metasomatized lithospheric mantle, and that the early intermediate to felsic magmas with adakite-like signatures formed from mafic magmas through strong amphibole fractionation without plagioclase in the lower crust. The late mafic magmas seem to be derived from a slightly different metasomatized lithospheric mantle by lower degrees of partial melting.

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