The role and significance of juvenile sediments in the formation of A-type granites, West Junggar oceanic arc (NW China): Zircon Hf-O isotopic perspectives

2020 ◽  
Author(s):  
Jiyuan Yin ◽  
Wenjiao Xiao ◽  
Christopher J. Spencer ◽  
Min Sun ◽  
Wen Chen ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Late Carboniferous ◽  
Ridge Subduction ◽  
Central Asian ◽  
Southern Central ◽  
Sedimentary Flux ◽  
West Junggar ◽  
O Isotope ◽  
High Alkali ◽  
T Values

Oceanic arc subduction systems are the loci of substantial recycling of oceanic crust and production of juvenile arc crust that differentiates to more evolved felsic crust. Inevitably, some juvenile sediments are subducted with the oceanic crust. However, distinguishing the incorporation of juvenile sediments in oceanic arcs is not always straightforward, because they may not measurably shift many geochemical signatures, such as Sr and Nd isotopes, of oceanic arcs. Nevertheless, combined zircon U-Pb, Hf, and O isotope data can provide a powerful tool to decipher sedimentary flux into oceanic arc magmas, and here we report a case study for the late Paleozoic A-type granites from the West Junggar oceanic arc in the southern Central Asian Orogenic Belt. These plutons contain hastingsite and iron biotite diagnostic minerals and have high alkali, FeOT/MgO, Zr, and Ga/Al, but possess low CaO contents, and strongly negative Eu, Sr, and Ba anomalies, demonstrating their close affinity with A-type granites. Zircon U-Pb analyses indicate that these A-type granites emplaced in the Late Carboniferous to Early Permian (ca. 307−298 Ma). Their high zircon εHf(t) values (+12.4 to +15.5), suggest that the magmas were derived from a mantle or juvenile crustal source. However, their δ18Ozrn (+7.2‰ to +11.9‰) values are significantly higher than that of the mantle, and modeling using Hf-O isotope and rare earth element data indicate the assimilation of sedimentary materials at a proportion of ∼50%. Our data suggest that juvenile sediments (e.g., greywacke) played an important role in the formation of the studied A-type granites. The re-melting of sedimentary material induced by the late Carboniferous ridge subduction can promote the transition from an intra-oceanic arc to continental crust. Our results show that the subduction and re-melting of juvenile sediments in oceanic arc systems could be an important mechanism for the maturation of oceanic arc crust.

Geochemical, Sr–Nd–Pb and zircon U–Pb–Hf isotopic constraints on the Late Carboniferous back-arc basin basalts from the Chengjisihanshan Formation in West Junggar, NW China

2020 ◽  
Vol 157 (11) ◽  
pp. 1781-1799
Author(s):  
Qian Zhi ◽  
Yongjun Li ◽  
Fenghao Duan ◽  
Lili Tong ◽  
Jun Chen ◽  
...  
Keyword(s):  
Volcanic Rocks ◽  
Oceanic Lithosphere ◽  
Late Carboniferous ◽  
Geochemical Data ◽  
Upper Carboniferous ◽  
Basaltic Rocks ◽  
Central Asian ◽  
West Junggar ◽  
Back Arc ◽  
T Values

AbstractWest Junggar in the southwestern Central Asian Orogenic Belt is a critical area for the study of the Junggar oceanic basin and may also reveal tectonic evolutionary events before the final closure of the Palaeo-Asian Ocean. The sedimentary formations and paragenetic associations of the Upper Carboniferous Chengjisihanshan Formation in southern West Junggar jointly reveal a back-arc basin setting with zircon U–Pb ages of 313–310 Ma for the basaltic rocks. Geochemically, the basaltic rocks are tholeiitic with low SiO2 (47.76–52.06 wt %) and K2O (0.05–0.74 wt %) but high MgO (6.55–7.68 wt %) contents and Mg no. (52.9–58.9) values. They display slightly flat rare earth element patterns with weak positive Eu anomalies, and show enrichments in large ion lithophile elements relative to high field strength elements with negative Nb and Ta anomalies, exhibiting both N-MORB-like and arc-like signatures, similar to the back-arc basin basalt from the Mariana Trough. The high positive zircon εHf(t) and bulk εNd(t) values as well as high initial Pb isotopes, together with relatively high Sm/Yb and slightly low Th/Ta ratios imply a depleted spinel lherzolitic mantle source metasomatized by slab-derived fluids. The field and geochemical data jointly suggest that the volcanic rocks within the Chengjisihanshan Formation were formed in an intra-oceanic back-arc basin above the northwestward subduction of the Junggar oceanic lithosphere in southern West Junggar. The confirmation of the Late Carboniferous back-arc basin basalts, together with other geological observations, indicate that an arc-basin evolutionary system still existed in southern West Junggar at c. 310 Ma, and the Junggar Ocean closed after Late Carboniferous time.

scholarly journals AGE, PETROGENESIS AND TECTONIC IMPLICATIONS OF THE LATE PERMIAN PERALUMINOUS AND METALUMINOUS MAGMATIC ROCKS IN THE MIDDLE GOBI VOLCANOPLUTONIC BELT, MONGOLIA

2021 ◽  
Author(s):  
Ariuntsetseg Ganbat ◽  
Tatsuki Tsujimori ◽  
Laicheng Miao ◽  
Inna Safonova ◽  
Daniel Pastor-Galán ◽  
...  
Keyword(s):  
Active Continental Margin ◽  
Biotite Granite ◽  
Ridge Subduction ◽  
The North ◽  
Magmatic Rocks ◽  
Central Asian ◽  
Isotopic Analyses ◽  
History Of ◽  
Arc Setting ◽  
T Values

The Mongol–Okhotsk Belt, the youngest segment of the Central Asian Orogenic Belt, formed by the evolution and closure of the Mongol–Okhotsk Ocean. The oceanic closure formed two volcanoplutonic belts: Selenge Belt in the north and Middle Gobi Belt in the south (in present day coordinates). However, the origin and tectonic evolution of the Mongol–Okhotsk Belt in general, the origin and formation age of the Middle Gobi Belt in particular, remain enigmatic. To better understand the history of the magmatic activity in the Middle Gobi Belt, we conducted geochemical, U–Pb geochronological, zircon Hf, whole-rock Nd isotopic analyses of volcanic and plutonic rocks of the Mandalgovi suite, the major component of the Middle Gobi Belt. Our results show that the Mandalgovi suite consists of (i) 265 ± 2 Ma biotite-granite; (ii) 250 ± 3 Ma hornblende-granitoids; (iii) their volcanic equivalents of both: and (iv) gabbro-diorites. The geochemical compositions indicate that their precursor magmas were derived from crustal source. The protoliths of the biotite and hornblende-granitoids were metagraywacke and metabasalt, respectively. They are characterized by positive whole-rock εNd(t) and zircon εHf(t) values, indicating the molten protoliths were juvenile crust. The biotite-granites formed by remelting of fore-arc sediments by ridge subduction and later hornblende-granites were emplaced at an intra-oceanic arc by the subduction of the Mongol–Okhotsk Ocean. We conclude that the magmatic rocks of the Middle Gobi formed in an active continental margin and/or intra-oceanic arc setting.

Tectonic and magmatic evolution of the Aqishan-Yamansu belt: A Paleozoic arc-related basin in the Eastern Tianshan (NW China)

2020 ◽  
Author(s):  
Shuanliang Zhang ◽  
Huayong Chen ◽  
Pete Hollings ◽  
Liandang Zhao ◽  
Lin Gong
Keyword(s):  
Partial Melting ◽  
Oceanic Crust ◽  
Lower Crust ◽  
Early Carboniferous ◽  
Late Carboniferous ◽  
Igneous Rocks ◽  
Calc Alkaline ◽  
Eastern Tianshan ◽  
Depleted Mantle ◽  
T Values

The Aqishan-Yamansu belt in the Chinese Eastern Tianshan represents a Paleozoic arc-related basin generally accompanied by accretionary magmatism and Fe-Cu mineralization. To characterize the tectonic evolution of such an arc-related basin and related magmatism and metallogenesis, we present a systematic study of the geochronology, whole-rock geochemistry, and Sr-Nd isotopes of igneous rocks from the belt. New zircon U-Pb ages, in combination with published data, reveal three phases of igneous activity in the Aqishan-Yamansu belt: early Carboniferous felsic igneous rocks (ca. 350−330 Ma), late Carboniferous intermediate to felsic igneous rocks (ca. 320−305 Ma), and Permian quartz diorite and diorite porphyry dikes (ca. 280−265 Ma). The early Carboniferous felsic rocks are enriched in large ion lithophile elements (LILEs) and depleted in Nb, Ta, and Ti, showing arc-related magma affinities. Their positive εNd(t) values (3.3−5.9) and corresponding depleted mantle model ages (TDM) of 0.83−0.61 Ga, as well as high MgO contents, Mg# values, and Nb/Ta ratios, suggest that they were derived from lower crust with involvement of mantle-derived magmas. The late Carboniferous intermediate igneous rocks show calc-alkaline affinities, exhibiting LILE enrichment and high field strength element (HFSE) depletion, with negative Nb and Ta anomalies. They have high MgO contents and Mg# values with positive εNd(t) values (3.9−7.9), and high Ba/La and Th/Yb ratios, implying a depleted mantle source metasomatized by slab-derived fluids and sediment or sediment-derived melts. The late Carboniferous felsic igneous rocks are metaluminous to peraluminous with characteristics of medium-K calc-alkaline I-type granites. Given the positive εNd(t) values (6.3−6.6) and TDM ages (0.56−0.53 Ga), we suggest the late Carboniferous felsic igneous rocks were produced by partial melting of a juvenile lower crust. The Permian dikes show characteristics of adakite rocks. They have relatively high MgO contents and Mg# values, and positive εNd(t) values (7.2−8.5), which suggest an origin from partial melting of a residual basaltic oceanic crust. We propose that the Aqishan-Yamansu belt was an extensional arc−related basin from ca. 350 to 330 Ma; this was followed by a relatively stable carbonate formation stage at ca. 330−320 Ma, when the Kangguer oceanic slab subducted beneath the Central Tianshan block. As the subduction continued, the Aqishan-Yamansu basin closed due to slab breakoff and rebound during ca. 320−305 Ma, which resulted in basin inversion and the emplacement of granitoids with contemporary Fe-Cu mineralization. During the Permian, the Aqishan-Yamansu belt was in postcollision extension stage, with Permian adakitic dikes formed by partial melting of a residual oceanic crust.

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