We present in situ trace element and Nd isotopic data of apatites from metamorphosed and metasomatized (i.e., altered) and unaltered granitoids in the Songnen and Jiamusi massifs in the eastern Central Asian Orogenic Belt, with the aim of fingerprinting granitoid petrogenesis, including both the magmatic and post-magmatic evolution processes. Apatites from altered granitoids (AG) and unaltered granitoids (UG) are characterized by distinct textures and geochemical compositions. Apatites from AG have irregular rim overgrowths and complex internal textures, along with low contents of rare earth elements (REEs), suggesting the re-precipitation of apatite during epidote crystallization and/or leaching of REEs from apatite by metasomatic fluids. εNd(t) values of the these apatites are decoupled from zircon εHf(t) values for most samples, which can be attributed to the higher mobility of Nd as compared to Sm in certain fluids. Apatites from UG are of igneous origin based on their homogeneous or concentric zoned textures and coupled Nd-Hf isotopic compositions. Trace element variations in igneous apatite are controlled primarily by the geochemical composition of the parental melt, fractional crystallization of other REE-bearing minerals, and changes in partition coefficients. Sr contents and Eu/Eu* values of apatites from UG correlate with whole-rock Sr and SiO2 contents, highlighting the effects of plagioclase fractionation during magma evolution. Apatites from UG can be subdivided into four groups based on REE contents. Group 1 apatites have REE patterns similar to the host granitoids, but are slightly enriched in middle REEs, reflecting the influence of the parental melt composition and REE partitioning. Group 2 apatites exhibit strong light REE depletions, whereas Group 3 apatites are depleted in middle and heavy REEs, indicative of the crystallization of epidote-group minerals and hornblende before and/or during apatite crystallization, respectively. Group 4 apatites are depleted in heavy REEs, but enriched in Sr, which are features of adakites. Some unusual geochemical features of the apatites, including the REE patterns, Sr contents, Eu anomalies, and Nd isotopic compositions, indicate that inherited apatites are likely to retain the geochemical features of their parental magmas, and thus provide a record of small-scale crustal assimilation during magma evolution that is not evident from the whole-rock geochemistry.
Earth science studies have focused on the deep mantle carbon cycle and its geodynamic effects. However, the way in which carbonated silicate melts modify the mantle remains poorly constrained. In this study, we report petrographic, mineral major and trace element, and Sr isotopic data for a suite of peridotite and pyroxenite xenoliths from the Shulan and Yitong areas of Northeast China. These data are used to investigate how carbonated silicate melts modify the mantle. The olivine, orthopyroxene, and clinopyroxene in the Shulan harzburgite and lherzolite xenoliths have relatively high Mg# values (90−93), and the clinopyroxenes have low (La/Yb)N ratios (0.49−0.61), implying that they may be post-partial melting residues of mantle previously modified by silica-rich melts, which is consistent with the light rare earth element (LREE)-depleted REE patterns of the clinopyroxenes. The olivine, orthopyroxene, and clinopyroxene in the Shulan (olivine) websterite xenoliths also have high Mg# values (90−94) that are indicative of a mantle origin. However, the clinopyroxenes have high (La/Yb)N (1.43−65.8), Ti/Eu (3504−5255), and Ca/Al (5.13−9.59) ratios, a positive correlation between Sm/Hf and Zr/Hf ratios, and high LREE contents, which suggest carbonated silicate metasomatism. This inference is also consistent with the replacement of olivine and orthopyroxene by clinopyroxene. In contrast, the Mg# values (78−86) of the olivines in the Yitong orthopyroxenite, wehrlite, websterite, and clinopyroxenite xenoliths are lower than those of the corresponding minerals from peridotites. This result, combined with the variably LREE-enriched and high field strength element (HFSE)−depleted patterns (e.g., Nb, Ta, Zr, and Hf) of the clinopyroxenes, suggest that these pyroxenites could have crystallized from mantle-derived melts. Compared with the Shulan peridotites, the Yitong xenoliths record higher temperatures (1028−1310 °C) and higher (La/Yb)N (1.05−5.30) and Ti/Eu (2624−6567) ratios, and a positive correlation between La and Sr, which reflect the occurrence of carbonated silicate melts in the lithospheric mantle. This is also supported by the estimated equilibrium melt compositions obtained from clinopyroxene. Thus, we propose that the different types of Yitong xenoliths are the crystallized products of silica-rich to carbonated silicate melts. The low 87Sr/86Sr ratios (0.70345−0.70488) of the clinopyroxenes in the wehrlites and pyroxenites from Shulan and Yitong, along with the timing of formation of the Northeast Asian big mantle wedge (ca. 20 Ma) and geochemical characteristics of the Cenozoic basalts and high-Mg andesites in Northeast China, indicate that the carbonated silicate melts were derived from the partial melting of recycled carbonatized oceanic crust and ancient recycled crustal material in the mantle transition zone. This implies that the lithospheric mantle along the Yilan−Yitong faults experienced not only crystallization and the accumulation of silica-rich and carbonated silicate melts that formed the Yitong wehrlite and pyroxenite xenoliths, but also subsequent modification by carbonated silicate melts that formed the Shulan (olivine) websterite xenoliths. These mantle xenoliths record deep carbon cycling triggered by the subduction of the Pacific Plate.
This paper presents new zircon U–Pb ages and Hf isotope data, and whole‐rock major and trace element geochemical data of Permian igneous rocks from the eastern Songnen Massif (SM) and the western Jiamusi Massif (JM), NE China, to constrain their tectonic evolution. Zircon U–Pb dating indicates that Permian magmatism within the eastern SM can be subdivided into two stages: early Permian ( ca . 293 Ma) and middle to late Permian (272–257 Ma). The early Permian igneous rocks comprise a bimodal association of rhyolites, basaltic andesites, and gabbros. The middle to late Permian igneous rocks are dominated by gabbro–diorites, monzodiorites, quartz diorites, monzonites, quartz monzonites, granodiorites, monzogranites, syenogranites, and alkali feldspar granites, along with minor bodies of A‐type quartz trachytes. The Permian felsic magmas originated mainly from partial melting of a relatively juvenile lower crust, whereas the coeval mafic rocks were probably derived from partial melting of a relatively depleted lithospheric mantle that was modified by fossil subduction‐related fluids. The igneous rock associations and their geochemical features, together with data from coeval sedimentary and volcanosedimentary successions, indicate that Permian magmatism within the eastern Songnen and western Jiamusi massifs formed in an extensional environment, similar to a back‐arc setting, related to westward subduction of a Palaeo‐oceanic plate beneath the eastern margin of the Jiamusi Massif. We find no evidence for double‐sided subduction of the Mudanjiang oceanic plate during the Permian.
There is general agreement that a series of East Asian blocks has always lain outboard of both India and Australia along the North Indo−Australie peripheral orogen. However, whether the East Asian blocks were involved in the interior orogens of East Gondwana remains equivocal. The geochronology and geochemistry of Neoproterozoic−Late Triassic rocks in the Russian Far East, together with existing paleontological and detrital zircon data, offer an opportunity to determine the tectonic origin and drift history of the Bureya−Jiamusi−Khanka superterrane. Biotite and amphibole 40Ar/39Ar dating results define a distinctive episode of Late Pan-African (ca. 550 Ma) metamorphism and a local Late Triassic (ca. 219−200 Ma) episode of deformation for the Bureya−Jiamusi−Khanka superterrane. Zircon U−Pb ages and whole-rock geochemical data indicate that the Early Ordovician (483 ± 3 Ma) highly fractionated I-type monzogranites were emplaced in a post-collisional setting linked to the collapse of a Late Pan-African orogen, while the Late Triassic (ca. 234−223 Ma) A-type quartz syenites and I-type granite aplite dikes were formed in a slab-pull−induced passive continental margin of the subducting Mudanjiang oceanic plate. These crucial archives, complemented by data from the literature, reveal that the Bureya−Jiamusi−Khanka superterrane made up the northernmost Kuunga-Pinjarra interior orogen during the final assembly of East Gondwana. As a result of Devonian rifting after Early Ordovician orogen collapse, the Bureya−Jiamusi−Khanka superterrane escaped from the Kuunga-Pinjarra interior orogen and subsequently migrated to Northeast Asia by the Late Triassic to Jurassic due to the subduction and closure of the Paleo-Tethys and Paleo-Pacific oceans.
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