查看更多>>摘要：This study presents a comprehensive analysis of zircon U-Pb-Hf isotopic and whole-rock geochemical data of Carboniferous-Early Permian felsic igneous rocks from the Uliastai continental margin (UCM), southeastern Central Asian Orogenic Belt, to constrain the amalgamation of the Inner Mongolia-Daxing'an Orogenic Belt (IMDOB) in the late Paleozoic. Zircon laser ablation inductively coupled plasma mass spectrometry U—Pb ages of monzogranite, granite porphyry, felsic volcanic rock, and alkali feldspar granite reveal three stages of magmatism in the Early Carboniferous (ca. 336-320 Ma), Late Carboniferous (ca. 311-307 Ma), and Early Permian (ca. 298 Ma). The Early Carboniferous igneous rocks, including monzogranites and later granite porphyries, exhibit affinity with the high-K calc-alkaline highly fractionated I-type granite, and are depleted in high-field-strength elements and enriched in large-ion lithophile elements. Their zircon ensit) values of the Early Carboniferous rocks range from +4.6 to +9.1, indicating that they were generated by partial melting of the juvenile crustal materials in a north-dipping subduction-related environment along with the opening of the Hegenshan Ocean. The Late Carboniferous felsic volcanic rocks are highly fractionated high-K calc-alkaline to alkali-calcic I-and A-type granitoids, with relatively strong peraluminous affinities. Considering the zircon ￡hK0 values (+6.2 to +10.6) and relatively low Rb/Ba (0.71-3.57) and Rb/Sr (4.18-8.12) ratios, they were likely derived from a juvenile crust and are comparable to partial melts of clay-poor but plagioclase-rich metaigneous rocks. Notably, the A-type granitoids are gradually increasing in the UCM after 311 Ma, and these magmatic rocks have markedly increased zircon saturation temperatures and (K2O + Na2O)/CaO ratios. These changes may result from the slab break-off of the Hegenshan Ocean, and the Late Carboniferous felsic volcanic rocks were formed in a post-collisional setting. The Early Permian alkali feldspar granites are characterized by typical A-type granite geochemistry, with high Ga/Al ratios; low MgO, Cr, Co, and Ni contents; and high zircon saturation temperatures. These rocks have positive eHf(t) values of +10.2 to +12.7 and record an intracontinental extension setting. In combination with the regional geology, the Late Carboniferous post-collisional magmatic rocks suggest that a change from a subduction-related continental margin to a post-collisional tectonic regime might have occurred in the UCM before 311 Ma.
查看更多>>摘要：Magmatic rocks from intra-oceanic arcs are critical for understanding the formation of continental crust and tectonic evolution. The early tectonic evolution of the Neo-Tethyan Ocean before the final Indo-Asia collision remains mysterious, and the geodynamic processes that triggered the Cretaceous magmatism in central Myanmar is still debated. The Cretaceous magmatic complex in the Banmauk-Kawlin area (BKC), west Myanmar terrane (WMT) is composed of the Kanza Chaung granitoid batholith, the Mawgyi Volcanic rocks, and the Pinhinga plutonic complex. Zircon U-Pb dating results of various rocks from die Kanza Chaung batholith suggest magmatism lasted from ca. 110 to ca. 94 Ma, roughly overlapping with new geochronological data for the Mawgyi Volcanics. Mafic rocks, including basalts from the Mawgyi Volcanics and gabbros from die Kanza Chaung Batholith, have geochemical features resembling intra-oceanic arc magmas, characterized by high large-ion-lithophile elements (LILEs) and low high-field-strength elements (HFSEs) and flat trace element patterns. They have depleted Sr (initial 87Sr/86Sr = 0.7035-0.7054) and Nd (eNd(t) = 0.39-6.71) isotopic compositions, with zircon eHf(t) values ranging from +5.8 to +16.1, probably derived from partial melting of the mantle wedge. Diorites formed by differentiation of basaltic magma have similar trace element patterns and Sr-Nd isotopes. The granitic rocks were likely originated from partial melting of juvenile arc lower-crust, indicated by their high SiO2 O65.0 wt%), low MgO (<2.50 wt%) and depleted Nd and zircon Hf isotopes. The eNd(t) values of the BKC shift markedly (from —r 7 to 0) from 105 to 94 Ma, which correlates with a temporal increase of Th/Nb, La/Ta, and La/Sm. Given the juvenile characteristics of the WMT crust, this can be explained by exotic isotopically enriched crustal components subducted into the mantle source, rather than steady-state sediment subduction and crustal contamination. Given the Albian unconformity in the WMT and recent paleomagnetic data, such continent crustal components were likely introduced by collision followed by subduction of Greater India-derived continental sliver beneath the WMT. Thus crust with an Indian continent affinity was possibly accreted to an intra-oceanic arc (WMT) during the mid-Cretaceous.
查看更多>>摘要：China's Chang'E-5 mission returned new samples from the Moon and has extended the eruption ages of lunar volcanism ca. 800 million years younger than previous determinations. This finding challenges our past perceptions of the lunar thermal and magmatic evolution and has attracted great attention. Since the lunar basalts show a wide compositional range, it is essential to classify them carefully in order to define their origin and evolution. However, whether the Chang'E-5 basalts are classified as high-Ti or low-Ti type remains debatable because the basalt clasts picked from the Chang'E-5 soil samples are so tiny that tiieir bulk composition estimations involve large uncertainties. Instead of using bulk composition, this study employs Ti in olivine to track the crystallization sequence of the Chang'E-5 basalts and then to classify them. In the Chang'E-5 olivines, Ti contents first increase and then decrease with a continuous decrease of forsterite values. Such a compositional variation reveals a magma differentiation series from early ilmenite unsaturation to ilmenite saturation during the late stage. Furthermore, the TiO2 content of the hypothesized parental magma that corresponds to the most primitive olivine is estimated to be —4.4 wt%. These results confirm that the Chang'E-5 basalts originated from a low-Ti primary magma. This study shows that olivine can be used as a robust mineral indicator to distinguish between high-Ti and low-Ti mare basalts.
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