Flat-slab subduction and formation of “intraplate” porphyry deposits: Insights from the Jurassic high and low La/Yb ore-forming porphyries along the Qin-Hang belt, South China

Abstract

The Middle-Late Jurassic porphyry deposits with different element associations in the interior of South China Block are over 1000 km away from subduction zone, which conflicts with the traditional belief that the porphyry Cu-Au-Mo deposits were formed in the convergent margin. However, their genesis and geodynamics are still under debate. To address the above issues, this paper reports new geochronological, mineralogical, elemental and isotopic data for the Yuanzhuding porphyry deposit from Qin-Hang belt, and compared with the published data of porphyry deposits in this belt. The Yuanzhuding porphyry deposit has zircon U-Pb and molybdenite Re-Os ages of 154.6–155.2 Ma and 156.3 Ma respectively, which suggest that it is contemporaneous with the Middle-Late Jurassic Dexing, Baoshan and Dabaoshan porphyry deposits in the belt. The Yuanzhuding and Dexing porphyries have adakitic signatures and Nd-Hf isotopes similar to the contemporaneous EMI-type-mantle-derived mafic rocks featuring high Nb, P2O5 and LILEs/HFSEs values, suggestive of their roots in EMI-type lithospheric mantle hybridized by basaltic-slab-derived melts and pelagic-sediments-derived fluids. Combined with the slightly lower Nd isotopes of Yuanzhuding porphyry than Dexing porphyry, the fractionation modeling suggests that they were formed via AFC and FC processes of basaltic magma respectively and that their adakitic signatures can be attributed to hornblende fractionation. In contrast, the Baoshan and Dabaoshan porphyries have high Yb contents, low (La/Yb)N ratios and Nd-Hf isotopes similar to the Proterozoic crust in the region. Mineral and whole-rock elemental plots reveal that they were stemmed from FC processes of partial melts of the Proterozoic crust, with involvement of voluminous and a little EMI-type-mantle-derived basaltic magma respectively. Integrated with the spatio-temporal evolution of Triassic-Jurassic magmatic, metamorphic, sedimentary, tectonic and structural records in the region, the petrogenesis of ore-forming porphyries indicates that the porphyry deposits were controlled by the northwestward flat-slab subduction and related slab break-off. The basaltic magma sourced from the EMI-type mantle had high Cu, H2O and S contents and oxidized signature, which are favorable for the formation of Dexing porphyry Cu-Au-Mo mineralization via differentiation and volatile exsolution in a closed magma chamber. Also, it can incorporate crustal materials via AFC processes, thus forming Yuanzhuding porphyry Cu-Mo mineralization. In addition, the thick crust caused by flat-slab subduction facilitated the input of voluminous and a little basaltic magma into crustal Baoshan and Dabaoshan porphyries, thus generating Pb-Zn-Cu-Mo and Mo-W-Pb-Zn mineralization, respectively. We, therefore, propose that the “intraplate” porphyry deposits with different element associations result from complicated crust-mantle interaction in the thick “intraplate” crust, and that their metallogenic elements will change from Cu-Au-Mo- to Mo-W-Pb-Zn-dominated mineralization with the transformation of magma sources from enriched-mantle- to continental-crust-dominated materials.