Identification of the Shuanglongtan tectonic mélange (ca. 1.7–1.5 Ga) in the southwestern Yangtze Block and it’s tectonic implications

Abstract

The Yangtze Block hosts large quantities of late Paleoproterozoic to early Mesoproterozoic mafic rocks, which are considered to be related to the breakup of the Nuna supercontinent. This study identified a tectonic mélange suite within the Shuanglongtan fault zone (also referred to as the Shuanglongtan tectonic mélange) in the southwestern Yangtze Block. Field observations and rock assemblages suggest there are five litho-tectonic units within the tectonic mélange: ultramafic rocks (pyroxene peridotites and picrites), mafic dikes (gabbros and diabases), meta-basalts and pelagic sediments, ocean-island basalts (OIBs) and seamount sediments (marbleized limestones), and exotic blocks (grayish-purple metasandstones and siltstones with hematite mineralization). These litho-tectonic units experienced greenschist to low amphibolite facies metamorphism, as revealed by the mylonitized and lineated structures. The zircon U-Pb dating indicates that these mafic volcanic rocks within the tectonic mélange were formed from the late Paleoproterozoic to the early Mesoproterozoic (1728 ± 27 to 1493 ± 12 Ma). These rocks, formed by the 5 %–15 % partial melting of a mantle source composed of spinel-garnet lherzolites, can be classified into enriched mid-ocean ridge basalt (E-MORB)-like and OIB-like meta-basalts based on their geochemical features. In combination with previous studies and current observations, this study proposes that the Shuanglongtan tectonic mélange emerges as the eastward extension of the Caiziyuan ophiolite mélange and that the Yangtze Block was divided into the eastern and western portions by an oceanic basin from the late Paleoproterozoic to the early Mesoproterozoic.