Arc-like volcanic rocks from the southern Lancangjiang zone, SW China: Geochronological and geochemical constraints on their petrogenesis and tectonic implications

Abstract

Triassic volcanic rocks are mainly distributed in the central part of the San-Jiang orogenic belt in southwestern China, and are exposed dominantly in the eastern segment of the Lancangjiang zone. Interpreted as the products of subduction during closure of the paleo-Tethyan Ocean, these volcanic rocks are important for reconstructions of the evolution of the paleo-Tethyan Ocean in southwestern China. Petrologically, the Triassic volcanic rocks in the zone are dominated by andesites with minor basaltic andesites. An andesite sample yielded a SHRIMP U–Pb zircon age of 248.5 ± 6.3 Ma, suggesting that these volcanic rocks erupted in the Early Triassic and not the Middle Triassic as previously assumed. Geochemically, these volcanic rocks have enrichments in LILE and LREE and depletions in HFSEs, significantly similar to typical subduction-related arc volcanic rocks. They are also characterized by relatively high Al₂O₃ contents and radiogenic Sr–Pb isotopic compositions, indicating that their mantle source could have experienced the modification of subducted sediments and slightly fluid/melt metasomatism from subducted pelagic sediments. The results of mixing calculations further confirm that the mantle source of the Early Triassic volcanic rocks in the area could have been predominantly modified by involvement of 5–7% subducted pelagic sediments into an Indian MORB-like mantle source. Associated with large volumes of Middle-Triassic (∼ 230 Ma) syn-collision granites in the region, these volcanic rocks in the southern Lancangjiang zone, southwestern China, are considered to have developed in a continental margin volcanic arc setting during Early Triassic time, which suggests that the paleo-Tethyan Ocean had not been closed until Middle Triassic when the continent–continent or continent–arc collision occurred.

Introduction

The San-Jiang orogenic belt (a collective name for the Nujiang, Lancangjiang and Jinshajiang belts in the Chinese literature) in western Yunnan Province, SW China, marks the suture between the Eurasian and Indian plates. The occurrence of Late Paleozoic ophiolitic suites, oceanic island and ocean ridge basalts along the Changning–Menglian orogenic belt, and the upper Paleozoic lithostrata and biostrata between the Yangtze and Baoshan blocks (Zhang et al., 1985, Zhang et al., 1988, Zhong, 1998, Jian et al., 2004, Feng et al., 2005), suggests that the paleo-Tethyan Ocean existed from at least the Middle Devonian. High-pressure glaucophane schists, arc igneous rocks (Zhang et al., 1985, Cong et al., 1993, Mo et al., 1998, Zhong, 1998), the Sukhothai–Chiang Mai–Lincang voluminous granitic zone, kilometer thick Triassic volcanic sequences, and Early-Mesozoic molasse sediments along the Changning–Menglian belt, further indicate that subduction of the paleo-Tethyan ocean and subsequent collision between the Yangtze and Baoshan blocks occurred during the Permian to Triassic times (Liu et al., 1989, Mahawat et al., 1990, Cong et al., 1993, Zhong, 1998, Jian et al., 2003, Peng et al., 2006).

Triassic igneous rocks are common along the Changning–Menglian orogenic belt. These rocks can provide important information on the nature of the lithosphere beneath the area and the tectonic evolution of the paleo-Tethyan Ocean. However, previous studies mainly focused on the origin of the belt of Triassic granitic rocks that extends from the northwestern Yunnan (China) to Thailand (e.g., YNBGMR, 1990, Liu, 1993, Peng et al., 2006). Little attention has been paid to the coeval volcanic rocks. Consequently, little is known about the age or composition of these rocks, or whether these rocks formed under a subduction-related island-arc environment or in a within-plate extensional setting.

In this paper, we present a new SHRIMP U–Pb zircon age, and elemental and Sr–Nd–Pb isotopic data for the volcanic rocks from the Lancangjiang zone in Jinghong county, western Yunnan. Our aims are to: (1) study the petrogenesis of these volcanic rocks and assess the nature of their mantle source, with an emphasis on the role of crust–mantle interaction; (2) provide new constraints on the temporal and spatial relationships among the Triassic volcanism, and (3) advance our understanding of the Early Triassic tectonic evolution of the paleo-Tethyan Ocean in western Yunnan, SW China.