Geochronology, geochemistry, and Sr-Nd-Pb-Hf isotopes of the late Paleozoic Dongshuigou composite pluton in the Dunhuang Block, southernmost Central Asian Orogen: Petrogenesis and implications for crustal growth

Abstract

Late Paleozoic granitoid rocks hold critical information regarding continental crustal growth and evolution of the Dunhuang Block at the southernmost margin of the Central Asian Orogenic Belt (CAOB). Here, we report new zircon U-Pb ages and Hf isotope data, as well as whole-rock geochemistry and Sr-Nd-Pb isotopic compositions for Devonian granitoid rocks and dacite porphyry in the Dongshuigou area of the Dunhuang Block, NW China. Petrographic investigation suggests that the Dongshuigou composite pluton, mainly consisting of quartz diorite, granodiorite, and biotite granite, was emplaced in the middle Devonian (ca. 390–380 Ma), whereas the dacite porphyry intruded into the granodiorite in the late Devonian (ca. 367 ± 4 Ma). The quartz diorite and granodiorite are similarly sodic, middle-K calc-alkaline, high Mg# (48.5–49.8) and Cr-Ni contents, enriched in LREE and LIIE (e.g., Rb, Ba, and P), and depleted in HFSE (e.g., Na and Ta), indicative of typically arc-like geochemical signatures. The quartz diorite has positive εNd(t) (+0.38), variable εHf(t) values (+3.12 to +10.7), juvenile Nd (1.04 Ga), and Hf model ages of 0.83 to 1.15 Ga. In comparison, the granodiorite displays negative εNd(t) values (−0.73) closed to the chondrite line, positive εHf(t) values (+3.75 to +8.36), juvenile Nd (1.12 Ga), and Hf model ages of 0.83 to 1.12 Ga. Both plutonic rocks show low initial Pb isotopes similar to those of typical lower crust-derived rocks. Our data indicate that the diorites and granodiorites were derived from partial melting of the juvenile lower crust with addition of minor mantle materials. The high-K, calc-alkaline dacite porphyry exhibits relatively high SiO2 and K2O contents, moderate Mg# (45.2–46.2) values, enrichment in Rb, U, Pb, and K, depletion in Nb, Ta, and Ti, and low Sr and Y abundance. These geochemical characteristics are similar to those of lower crust-derived arc-related rocks, including the development of abundant inherited zircons, implying that the dacite porphyry was likely generated by a mixed source of the mafic lower crust and minor metasomatized mantle materials. The biotite granites are characterized by high Sr contents (481–542 ppm) and Sr/Y ratios (76.0–98.4), low Y (5.37–6.51 ppm) and Yb (0.50–0.56 ppm) contents, and high Mg# values (55–56) and Cr-Ni contents, showing geochemical affinities of adakitic rocks. They have positive εNd(t) (+2.97 to +3.00), variable εHf(t) values (−6.05 to +12.4; mostly >0) with Nd and average Hf model ages of 0.82 Ga and 0.92 Ga, respectively. The biotite granites exhibit low initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.336–18.339, 15.622–15.622, and 38.478–39.510, respectively, indicating that some subduction-related materials, such as mature arc primitive or ocean island volcanic rocks could have been added to the magma source. The biotite granites were likely derived from partial melting of subducted oceanic crust, which was modified by interactions with mantle wedge, with the addition of minor Mesoproterozoic materials during magma ascent. By synthesizing with the regional geology, we suggest that the middle Devonian granitoids and the late Devonian dacite porphyry in the Dongshuigou are arc/subduction-related rocks that formed in a continental arc setting. Taken together, Devonian arc-related magmatic events in the Dunhuang Block indicate significant crustal growth that was accompanied with minor reworking of ancient lower crust.