Abstract
The boundary between the Gondwana and Yangtze plate is still controversial. In southwest China, the Sanjiang region marks the collision zone which accreted several blocks coming from the northern Gondwana margin. In this region, subduction of the Paleo-Tethys Ocean and associated continental blocks during the Triassic Period led to the formation of an N–S trending complex involving intrusive and volcanic rocks. The intrusive rocks are important for constraining the evolution of the Paleo-Tethyan in southwestern China. This study presents new geochronological, geochemical, and Sr-Nd-Hf isotopic data of granite porphyries from northern Lancangjiang, in order to discuss the origin of these granites and their tectonic significance. Representative samples of the Zengudi and the Tuobake granite porphyries from the Yezhi area yielded weighted mean 206Pb/238U ages of 247–254 Ma and 246 Ma, respectively. The Zengudi granite porphyries display zircon ԐHf(t) values of −12.94 to −2.63, ԐNd(t) values of −14.5 to −9.35, and initial 87Sr/86Sr ratios of 0.708 to 0.716. The Tuobake granite porphyries have zircon ԐHf(t) values of −14.06 to −6.55, ԐNd(t) values of −10.9 to −9.41, and initial 87Sr/86Sr ratios of 0.716 to 0.731. Both the Zengudi and Tuobake granite porphyries exhibit strongly peraluminous signatures with high A/CNK nAl2O3/(K2O + Na2O + K2O) ratios (1.07–1.86 and 0.83–1.33, respectively). These granites are enriched in Rb and Th, and depleted in Ti, Nb, Ta, Sr, and P, with negative Eu anomalies (Eu/Eu* < 0.61). These geochemical and isotopic data indicate that the primary magma of the granite porphyries originated from partial melting of ancient continental crust as a result of basaltic magma underplating and underwent fractionation crystallization during their emplacement. We propose that the Triassic subduction of the Paleo-Tethys Ocean led to crust shortening and thickening in the Sanjiang region, while the northern Lancangjiang area was involved in the continental collision after the subduction of the Paleo-Tethys Ocean before 254 Ma.
Highlights
Three volcanic arc belts related to the subduction of the Paleo-Tethys Ocean were delineated in the Sanjiang region: the YushuYidun belt formed during Late Triassic (230–200 Ma) [25], the Jiangda-Weixi belt formed during Late Permian to Triassic (270–200 Ma) [21,36], and the Yunxian-Lvchun-Ailaoshan belt formed during Permian to Early–Middle Triassic (285–265 Ma and 260–230 Ma) [20,21]
Cathodoluminescence (CL) images of zircon grains were obtained at the Wuhan Sample Solution Analytical Technology Co., Ltd., using a scanning electron microscope coupled with an energy dispersive spectroscopy (EDS) system and a CL3+ detector under operating conditions of 15 kV and 4 nA
The U–Pb dating of the Zengudi and Tuobake granite porphyries from the Sanjiang area reveal that these granites were emplaced between 254 and 246 Ma, indicating that their emplacement occurred at the closure of the Paleo-Tethys Ocean during Late Permian to Early Triassic
Summary
The temporal evolution of magma in the arc volcanic belts of the Sanjiang region and the deep dynamic processes have been intensely studied and discussed [6,12,13,14,15,16,17,18,19,20,22,23], no consensus has been reached on the accurate timing and location of these volcanic arcs related to the Paleo-Tethys. We present new zircon U–Pb age and Hf isotopic data as well as whole-rock major, trace element, and Sr–Nd isotopic data of the Zengudi and Tuobake granite porphyries from the Yezhi area, aiming to: (1) constrain their formation age, (2) characterize their magma source and petrogenesis, and (3) investigate their tectonic setting and provide implications for the Paleo-Tethys evolution
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