Nature of early Mesoproterozoic magmatism in the Qilian-North Qaidam tectonic belt, NW China: Implications for prolonged extension processes and transition from Columbia to Rodinia

Abstract

Precambrian basement rocks exposed in uplift zones and orogenic belts provide accessible opportunities for paleogeographic reconstruction, especially for microcontinents and cratons with thick covers. Here we report early Mesoproterozoic rifting-related mafic and granitic metaigneous rocks preserved in the Qilian-North Qaidam tectonic belt, NE Tibetan Plateau. Mafic rocks include garnet amphibolite emplaced at 1.40 Ga and metagabbro that were emplaced at 1.39 Ga, exposed in the North Wulan area. They are derived from low-degree (<7 %) partial melting of slab-modified lithospheric mantle lherzolites with Sp:Grt ranging from 90:10 to 60:40. The calculated low P-T conditions of mantle melting (Tpmax < 1472–1451 °C at P = 2.0 Ga) are also consistent with a thermal normal lithospheric mantle upwelling event. Granitic rocks comprise ca. 1.44 Ga garnet-bearing and ca. 1.41 Ga two-mica granites that were emplaced in the North Wulan and Central Qilian areas, respectively. They are S-type granites derived from crustal reworking of psammitic-rich and/or clay-rich metasedimentary rocks. In addition, the two-mica granites represent the oldest basement rocks yet identified in Central Qilian and indicate the existence of ancient basement rocks older than ca. 1.41 Ga. All the mafic and granitic rocks belong to a part of intense Mesoproterozoic rift-related magmatism in the composite Qilian-Quanji block (QQB), and subsequently underwent 1.18–0.9 Ga metamorphic modifications. The composite QQB in the NE Tibetan Plateau was involved in the long-term processes including crustal reworking and mantle upwelling (1.5–1.35 Ga) correspond to the breakup of Columbia followed by oceanic subduction (1.2–1.0 Ga) and subsequent collision and exhumation (0.9–0.8 Ga) at the periphery of Rodinia. Our results improve the understanding of the Mesoproterozoic tectonic evolution for the whole QQB and provide important constrains on the transition from Columbia to Rodinia.