Constructing the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western Bainaimiao arc terrane, southeastern Central Asian Orogenic Belt

摘要

Identifying the crust-mantle interactions in association with the evolution of the Precambrian microcontinents provides critical constraints on the accretionary evolution in the Central Asian Orogenic Belt (CAOB). The Bainaimiao arc terrane (BAT) is one of the most important Precambrian microcontinents in southeastern CAOB, however, few studies have paid attention to the types and the evolving processes of the crust-mantle interactions that occurred before its final accretion onto the northern North China Craton. This study presents an integrated study of geochronology, zircon Hf isotope and whole-rock geochemistry on the latest Neoproterozoic diabases and the Early Paleozoic arc intrusions in the western BAT. The latest Neoproterozoic (ca. 546 Ma) diabases display low SiO2(46.52–49.24??wt.%) with high MgO (8.23–14.41??wt.%), Cr (66–542??ppm) and Ni (50–129??ppm), consisting with mantle origin. Their highly negative zirconεHf(t) (?12.0 to ?24.7) and high Fe/Mn ratios (62.1–81.7) further indicate a significantly enriched mantle source. Considering that the BAT maybe initially separated from the Tarim Craton with a thickened crustal root, we propose that these diabases were generated through partial melting of an enriched lithospheric mantle source that had been hybridized by lower-crustal eclogites during foundering of the BAT lower crust. The Early Paleozoic (ca. 475–417??Ma) arc intrusions in western BAT can be divided into Periods I and II at approximately 450??Ma. The Period I (>450??Ma) intrusions contain abundant mafic minerals like hornblende and pyroxene, and show positive zirconεHf(t) (+1.5 to +10.9). They are predominantly medium-K calc-alkaline with broad correlations of SiO2versus various major and trace elements, which correlate well with the experimental melts produced by the fractional crystallization of primitive hydrous arc magmas at 7??kbar. We assume they were formed through mid-crustal differentiation of the mantle wedge-derived hydrous basaltic melts. By contrast, the Period II (≤450??Ma) intrusions are characterized by variable zirconεHf(t) (?15.0 to +11.5) with irregular variations in most major and trace elements, which are more akin to the arc magmas generated in an open system. The general occurrence of elder inherited zircons, along with the relatively high Mg# (>45) of some samples, call upon a derivation from the reworking of the previously subduction-modified BAT lower crust with the input of mantle-derived mafic components. In combination with the Early Paleozoic tectonic mélanges flanking western BAT, we infer that the compositional transition from Period I to II can be attributed to the tectonic transition from south-dipping subduction of Solonker ocean to north-dipping subduction of South Bainaimiao ocean in southeastern CAOB. The above results shed light not only on the latest Neoproterozoic to Early Paleozoic multiple crust-mantle interactions in western BAT, but also on the associated crustal construction processes before the final arc-continent accretion.