In-situ mineralogical interpretation of the mantle geophysical signature of the Gangdese Cu-porphyry mineral system

Highlights

• The phlogopites in Bairong lamprophyres reveal asthenospheric components.

• The asthenosphere upwelling is the main mechanism for heat and material exchange.

• The mafic magmas expelled volatiles and metals to form ore-bearing magmas.

Abstract

The lithosphere provides a reservoir for ore-forming elements introduced by repeated fertilisation during asthenosphere-lithosphere interaction by the slab subduction. Here we use the Gangdese copper metallogenic belt to show how mineralogical study can help to interpret these mantle signatures. We present insights into the in-situ geochronological, geochemical and whole-rock isotopic analyses of minerals from mantle-derived lamprophyres that intruded the Bairong Cu-porphyry deposit in the Gangdese metallogenic belt. Zircon U-Pb dating yields weighted-mean concordant ²⁰⁶Pb/²³⁸U ages of 11.69 ± 0.27 Ma to 12.41 ± 0.83 Ma. These zircons show juvenile ε_Hf (t) (+1.0 ∼ +10.9) and low δ¹⁸O (5.5‰ ∼ 7.7‰) values. Variable initial ⁸⁷Sr/⁸⁶Sr ratios (0.7073–0.7080), ε_Nd(t) values (−6.3 ∼ −5.0) and initial ¹⁸⁷Os/¹⁸⁸Os ratios (0.1382–0.1591) suggest hybridization between lithosphere mantle and asthenospheric melts. Phlogopites in the lamprophyres reflect partial dissolution and incipient melting with low Mg# values but high Al₂O₃ contents in core. And these phlogopites are surrounded by a zone with high Mg# values. We interpret this as superheating and hybridization of refractory lithosphere mantle and asthenosphere. The 3-D magnetotelluric and seismic signature of the Gangdese mineral belt reflect a mantle source region comprising asthenosphere, old refractory lithosphere, ponded mafic melts near the crust-mantle boundary, and higher-level magma chambers where more extensive mixing between crust- and mantle-derived melts occurs. The asthenospheric upwelling provides a mechanism for heat transfer and the expulsion of volatile elements. It is generating a new young, fertile lithosphere beneath the southeast Lhasa block, which has driven the mineralization processes in the Gangdese metallogenic belt.

Introduction

Most of the continental lithosphere formed in the Archaean eon (e.g, Griffin et al., 2009) and provides stability to Earth’s surface, which is critical for maintaining habitability. While providing stable continental blocks, the continental lithosphere is reshaped during tectonic processes by chemical and physical modification. This cratonic lithosphere may also provide a long-term stable reservoir for ore-forming elements introduced by repeated fertilization (e.g., Griffin et al., 2013, Tassara et al., 2017). Hence, this fertilization, and the exchange of mass and heat between asthenosphere and lithosphere, may be a major factor in the generation of large ore systems (e.g., Holwell, et al., 2019). Here we examine this exchange within the lithosphere beneath a major Miocene Cu-Au porphyry metallogenic belt in a continental-collision environment in the Lhasa block of southern Tibet.

The Lhasa block, situated at the front of the India-Asia collisional belt, is a good place to explore the exchange of mass and heat during continental convergence (e.g., Zhu et al., 2011). Recent geophysical observations have identified contrasts in the present-day thermal regime from west to east in the Lhasa block using seismic tomography (e.g., Liang et al., 2016), seismic anisotropy (Chen et al., 2015a, Chen et al., 2015b) and magnetotelluric (MT) methods (Wang et al., 2017). The geophysical signature of the Gangdese metallogenic system comprises significant low velocity zones, large time lags (1.0–2.0 s) of shear-wave splitting (SKS) travel-time residuals, and a deep conductor in the eastern part.

Lamprophyres and related ultra-potassic rocks (UPRs) occur over much of the Lhasa block (Li et al., 2020, Li et al., 2021). They represent low-volume melts, and carry vital information on the state of the continental root at the time of major mineralization (Fig. 1, e.g., Zhao et al., 2009, Huang et al., 2015). These erupted and/or injected mafic magmas also provide an opportunity to study the enrichment processes and the compositionally heterogeneous mantle sources associated with the generation of these unusual magmas (e.g., Tappe et al., 2005). Several studies of UPRs have indicated the involvement of a subcontinental lithospheric mantle (SCLM) with radiogenic Rb-Sr and Lu-Hf isotopes and unradiogenic Sm-Nd isotopes beneath the western part of the Lhasa block (Zhao et al., 2009). In this study, we use mantle-derived mafic lamprophyres in the Bairong porphyry deposit in the southeastern part of the Lhasa block (Fig. 1) to provide temporal constraints on the thermal and chemical evolution of the continental lithosphere to identify the mechanisms reflected in the geophysical signature of the mineralization of the Gangdese belt.