Geochronology, geochemistry, and Hf isotopic compositions of the Early Devonian Heihaibei granite in the East Kunlun Orogen, Northwest China

Abstract

The East Kunlun Orogen (EKO) is located in the western part of the Central Asian Orogenic Belt, and it records the Neoproterozoic–Early Devonian tectonic evolution involving subduction and extinction of the Proto-Tethys Ocean. Heihaibei granite is located in the South Kunlun belt within EKO. In this study, geochronology, geochemistry, and Hf isotope analysis of Heihaibei monzogranite and granodiorite were carried out. Their LA-ICP-MS zircon U-Pb results yielded ages of 414.1 ± 2.0 and 417.8 ± 2.0 Ma. The Heihaibei granite is peraluminous (A/CNK = 0.99–1.30), with high alkali contents (7.76–9.14 wt.%) and 10000 Ga/Al (2.60–3.38), FeO<sup>T</sup>/MgO (27.6–49.5), and Y/Nb ratios (1.93–2.93), suggesting that the Heihaibei granite is an A<sub>2</sub>-type granite. The Heihaibei granite samples have zircon εHf (t) values ranging from -6.3 to -0.3, which suggests that the Early Devonian granite was derived by partial melting of Mesoproterozoic (TDM<sub>2</sub> = 1797–1415 Ma) crust. When considering data on contemporaneous granites, geochronological and geochemical data for Heihaibei granite indicate that it was emplaced in a post-collisional extension environment following the closure of the Proto-Tethys Ocean. The Heihaibei granite was formed by extension of the southern subduction accretionary complex as part of the post-collisional setting during the closure of the Proto-Tethys Ocean.

Isotope Geochemistry of the Heihaibei Gold Deposit within the Kunlun River Area in the Eastern Kunlun Orogen in Northwest China and Its Metallogenic Implications

“…An EW-to WNW-striking >15 km long and >100 m wide monzogranite-granodiorite dyke (i.e., the Heihaibei granites) was intruded along the south of the Silurian-Ordovician Nachitai Group. We argued that this monzogranite-granodiorite dyke was classified as A-type granite [50] and was emplaced in the Early Devonian (416 Ma) in a post-collisional setting [50]), rather than in the Ordovician-Silurian (442 Ma [27]), 443 Ma [25], and 453 Ma [23]). The Triassic Hongshuichuan Formation feldspathic quartz sandstone and other minor Triassic sediments were deposited over older sedimentary rocks and the Heihaibei granites [24].…”

“…The δ 18 OVSMOW values (+9.2-+13.9‰) of quartz separates from quartz veins at the Heihaibei gold deposit are basically similar to those of the Phanerozoic orogenic gold deposits Six potassic feldspar separates from five Heihaibei granite wall rocks have invariable Pb isotope compositions, with 206 Pb/ 204 Pb ratios of 18.3532 to 19.4864, 207 Pb/ 204 Pb ratios of 15.6475 to 15.6812, and 208 Pb/ 204 Pb ratios of 37.1750 to 38.4598, indicating the initial lead isotope for the granite wall rocks when they were emplaced at ca. 416 Ma [50].…”

“…200 Ma, since they were intruded in the Early Devonian. Using the average U, Th, and Pb abundances of the Heihaibei granite [50] and the possible mineralization age (Triassic,ca. 200 Ma younger than the Early Devonian emplacement), the age (200 Ma)-corrected Pb isotopic ratios, roughly representing the Triassic Pb isotope compositions for the Early Devonian Heihaibei granitic wall rocks, can be estimated (initial Pb isotopes of K-feldspars, plus radiogenic Pb isotopes after ca.…”

“…To date, mineralization in the Heihaibei gold deposit, whose timing has not been determined directly by radiometric dating but must be later than the Early Devonian (416 Ma, [50]) of the monzogranite wall rocks. As mentioned above, the EKO has undergone at least two orogenies that are closely related to the consumption of the Neo-Proterozoic to Early Devonian Proto-Tethys Ocean, which lays along the M.KLF, and the Carboniferous to Late Triassic Paleo-Tethys Ocean, which lays along the S.KLF-AMF, respectively [3,31,37].…”

“…As mentioned above, the EKO has undergone at least two orogenies that are closely related to the consumption of the Neo-Proterozoic to Early Devonian Proto-Tethys Ocean, which lays along the M.KLF, and the Carboniferous to Late Triassic Paleo-Tethys Ocean, which lays along the S.KLF-AMF, respectively [3,31,37]. We have previously considered that the EKO during the Late Early Silurian-Middle Devonian (430-389 Ma) was in the post-collisional stage related to the closure of the Proto-Tethys Ocean [75], and that the A-type Heihaibei granites were exact products of post-collisional magmatic emplacement during the post-collisional stage [50]. The northward subduction of the Paleo-Tethys oceanic plate was not initiated until the Carboniferous [3,31,37].…”