Zircon U–Pb geochronological and Hf isotopic constraints on petrogenesis of Late Mesozoic intrusions in the southeast Hubei Province, Middle–Lower Yangtze River belt (MLYRB), East China

Abstract

During the Late Mesozoic, extensive magmatism and associated ore deposits were developed in the southeast Hubei Province, Middle–Lower Yangtze River metallogenic belt (MLYRB), East China. To understand the petrogenesis of the intrusions in the area, we performed in situ zircon U–Pb and Hf isotopic analyses on 14 samples from the Fengshangdong stock, and from the Echeng, Tieshan, Wangbaoshan–Jinshandian and Yangxin batholiths. Three major suites of intrusive rocks are recognized in the southeast Hubei Province: (1) 141–146Ma granodiorite and granite porphyries (associated with 143–144Ma porphyry–skarn Cu–Mo deposits) have enriched radiogenic Hf isotopic compositions (εHf (t)=−9.5 to −1.6), comparable to those of Cretaceous pyroxene diorites, trachyandesites, and basalts (εHf (t)=−8.9 to −1.3) in the MLYRB, which were derived from an enriched lithospheric mantle. (2) Voluminous 136–143Ma gabbros, diorites, and quartz diorites (associated with 137–144Ma skarn Cu–Fe, Fe–Cu, and Au–Cu deposits) have variable Hf isotopic compositions (εHf (t)=−19.1 to −5.4), indicating an origin via the hybridization of enriched mantle-derived mafic magma and variable amounts of ancient lower-crustal components. (3) 127–133Ma diorites, quartz diorites, and granites (associated with 132–133Ma skarn Fe deposits) characterized by a wide range of εHf (t) values (−28.2 to −6.4), suggest the assimilation of various ancient lower-crustal materials by enriched mantle-derived basaltic magma. The intrusions of Suites 1 and 2 are characterized by high Sr contents (329–2087ppm), and high Sr/Yb (36–1710) and (La/Yb)N ratios(17.6–172), but low Yb contents (0.45–1.82ppm) and do not show negative Eu anomalies (δEu=0.74–1.4). The intrusions have radiogenic Sr–Nd isotopic compositions (εNd (t)=−10.0 to −3.4 and (87Sr/86Sr)i=0.7054–0.7084). Compared with the intrusions of Suites 1 and 2, rocks of Suite 3 have relatively low Sr contents (60.5–327ppm), and low Sr/Yb (33.0–315) and (La/Yb)N ratios (2.28–30.0), but relatively high Yb contents (1.04–5.17ppm) and significant negative Eu anomalies (δEu=0.22–0.73). The rocks of Suite 3 are characterized by a wide range of radiogenic Sr–Nd isotopes with εNd (t) values of −12.5 to −4.9 and (87Sr/86Sr)i=0.7057–0.7085. The proposed magma sources, combined with the geochemical and Sr–Nd isotopic differences between these three suites of intrusive rocks, indicate that Late Mesozoic magmatic activity in the southeast Hubei Province, MLYRB was associated with lithospheric thinning induced by asthenospheric upwelling at that time.