Final Closure Of Paleo-Asian Ocean Research Articles

Subduction initiation of Paleo-Pacific Ocean and final closure of Paleo-Asian Ocean: constraints from Triassic granitoids in Jiamusi Block, NE China

ABSTRACT The process of transformation and superposition from the Paleo-Asian Ocean to the Paleo-Pacific Ocean tectonic systems have been a critical scientific issue. However, the final closure of the Paleo-Asian Ocean and the initial subduction time of the Paleo-Pacific Ocean, together with the conversion between them in Northeast China, still remain controversial. The Triassic granites in the Jiamusi Block are located in the key position of Northeast China, recording the conversion process of two tectonic domains. Through systematic geochronology and geochemical studies, it can provide important constraints for revealing the superposition and conversion mechanism of the Paleo-Asian Ocean and the Paleo-Pacific Ocean and the early subduction history of the Paleo-Pacific Ocean plate. In this study, the granites in Jiamusi Block were formed in the Early–Middle Triassic (250–245 Ma), characterized by high Si, rich alkali, poor Mg, and are obviously enriched in light rare earth elements and large ion lithophile elements, depleting in high field strength elements such as Nb, Ta, and Ti. They have typical characteristics of continental magmatic arc, deriving from partial melting of amphibolite facies and garnet amphibolite facies mafic lower crust. Combined with zircon Hf isotope results, zircon εHf(t) values are −6.52 to + 9.79, and the two-stage Hf model ages are 0.6–1.7 Ga, indicating that the magma should be derived from a relative younger crustal source, and some ancient materials may be added. Moreover, based on the spatial-temporal framework and Hf isotope characteristics of magmatism in Northeast China, we believe that the Jiamusi Block belonged to an exotic block. Meanwhile, combined with geochronological and geochemical data of igneous rocks on the Jiamusi Block, we suggest that the evolution of Jiamusi Block was dominated by the Paleo-Pacific Ocean rather than Paleo-Asian Ocean during the Triassic, which means the subduction of Paleo-Pacific Ocean plate initiated at least in Early Triassic.

Late Permian medium-pressure metamorphism in the eastern Songnen Massif, eastern Central Asian Orogenic Belt (NE China): Implications for the final closure of the Paleo-Asian Ocean

The nature of late Paleozoic metamorphism in the eastern Central Asian Orogenic Belt (CAOB) is controversial. This paper reports mineral chemistry, petrological, and zircon and titanite U–Pb age data for representative rocks in the Dongfengshan Group from the eastern margin of the Songnen Massif. We use these data to determine the rock associations, P–T conditions, and timing of metamorphism. The P–T conditions were estimated from 54 mineral pairs in representative rocks, including garnet–biotite schist, garnet–two-mica schist, garnet–biotite–plagioclase gneiss, and garnet amphibolite, and indicate the Dongfengshan Group experienced metamorphism at P = 5.5–7.1 kbar and T = 592–630 °C (i.e., a geothermal gradient ranging from 25 to 30 °C/km) under medium-pressure lower amphibolite-facies conditions. The biotite–plagioclase gneiss and garnet amphibolite yielded titanite U–Pb ages of 251 and 260 Ma, respectively, which are interpreted to represent the regional metamorphic age (260–251 Ma). The consistency of these dates and ages of metamorphic rims in zircons from a biotite plagiogneiss (252 Ma) confirm that medium-pressure metamorphism occurred in the late Permian rather than the Neoproterozoic, as assumed previously. Based on metamorphic, magmatic, stratigraphic, and paleontological constraints, as well as the regional tectonic history, we propose that late Permian medium-pressure metamorphism in the Dongfengshan Group occurred during the collision and amalgamation of the Siberian and North China cratons following the final closure of the eastern Paleo-Asian Ocean.

The Late Permian highly fractionated I-type granites from Sishijia pluton in southestern Inner Mongolia, North China: A post-collisional magmatism record and its implication for the closure of Paleo-Asian Ocean

Whole-rock major and trace elements, zircon UPb geochronology and Hf isotopic data have been implemented for the felsic rocks from the Sishijia pluton in southeastern Inner Mongolia, North China, in order to determine their petrogenesis and futher constrain the termination of Paleoasian Oceanic subduction in southeastern Central Asian Orogenic Belt. Petrographic observation indicates that Sishijia granitoids mainly consist of syenogranites and porphyritic granites, with the major mineral compositions of alkaline feldspar, quartz, plagioclase and biotite, without any alkaline melanocratic or peraluminous minerals. Miarolitic structures are widely developed in these granitoids, indicating for the characteristics of shallow emplacement depth and rapid condensation. Zircon LA-ICP-MS UPb dating yields weighted mean ages of 256.2 ± 1.4 and 250.9 ± 1.2 Ma for syenogranites, and 251.6 ± 1.4 Ma for porphyritic granites, representing a Late Permian magmatism record. These granitoids exhibit weakly peraluminous nature (A/CNK = 1.03–1.08) with high SiO2 (76.50–81.92 wt%) and K2O (3.64–5.56 wt%) concentrations and differentiation index values (D.I. = 93.8–97.2). Chondrite-normalized REE patterns of these samples display enrichment of LREE with high ratios of (La/Yb)N (12.29–30.34) and significant negative Eu anomalies (δEu = 0.15–0.53). In mantle-normalized multielement variation diagrams, they are all characterized by typical depletions of Ba, Nb, Ta, Sr, P, and Ti and enrichments of Rb, Th, U, and K. Petrological and geochemical features of the pluton indicate a highly fractionated I-type affinity. Zircon Hf isotope analyses show variably negative εHf(t) values ranging from −13.7 to −4.0 and corresponding two-stage model ages varying of 2148–1540 Ma, indicating that they were derived from an Paleo- to Mesoproterozoic crustal source with minor juvenile materials. In combination with previous geological evidences, it is reasonable to propose that the Sishijia granitoids emplaced in a post-collisional extension regime during the transformation from pre-collision to post-orogenic, which symbolize a Late Permian final closure of Paleo-Asian Ocean in southeastern segment of the Central Asian Orogenic Belt.