Timing and genesis of the adakitic and shoshonitic intrusions in the Laoniushan complex, southern margin of the North China Craton: Implications for post-collisional magmatism associated with the Qinling Orogen

Abstract

The NWW-striking Qinling Orogen formed in the Triassic by collision between the North China and Yangtze Cratons. Triassic granitoid intrusions, mostly middle- to high-K, calc-alkaline, are widespread in this orogen, but contemporaneous intrusions are rare in the southern margin of the North China Craton, an area commonly considered as the hinterland belt of the orogen. In this paper, we report zircon U–Pb ages, elemental geochemistry, and Sr–Nd–Hf isotope data for the Laoniushan granitoid complex that was emplaced in the southern margin of the North China Craton. Zircon U–Pb dating shows that the complex was emplaced in the late Triassic (228 ± 1 to 215 ± 4 Ma), indicating that it is part of the post-collisional magmatism in the Qinling Orogen. The complex consists of, from early to late, biotite monzogranite, quartz diorite, quartz monzonite, and hornblende monzonite, which span a wide compositional range, e.g., SiO 2 = 55.9–70.6 wt.%, K 2O + Na 2O = 6.6–10.2 wt.%, and Mg # of 24 to 54. The biotite monzogranite has high Al 2O 3 (15.5–17.4 wt.%), Sr (396–1398 ppm) and Ba (1284–3993 ppm) contents and relatively high La/Yb (mostly 14–30) and Sr/Y (mostly 40–97) ratios, but low Yb (mostly 1.3–1.6 ppm) and Y (mostly14–19 ppm) contents, features typical of adakitic rocks. The quartz monzonite, hornblende monzonite and quartz diorite have a shoshonitic affinity, with K 2O up to 5.58 wt.% and K 2O/Na 2O ratios averaging 1.4. The rocks are characterized by strong LREE/HREE fractionation in chondrite-normalized REE pattern, without obvious Eu anomalies, and show enrichment in large ion lithophile elements but depletion in high field strength elements (Nb, Ta, Ti). The biotite monzogranite (228 Ma) has initial 87Sr/ 86Sr ratios of 0.7061 to 0.7067, ε Nd(t) values of − 9.2 to − 12.6, and ε Hf(t) values of − 9.0 to − 15.1; whereas the shoshonitic granitoids (mainly 217–215 Ma) have similar initial 87Sr/ 86Sr ratios (0.7065 to 0.7075) but more radiogenic ε Nd(t) (− 12.4 to − 17.0) and ε Hf(t) (− 14.1 to − 17.0). The Sr–Nd–Hf isotope data indicate that the rocks were likely generated by partial melting of an ancient lower continental crust with heterogeneous compositions, as partly confirmed by the widespread presence of early Paleoproterozoic inherited zircons. Mafic microgranular enclaves (MMEs), characterized by fine-grained igneous textures and an abundance of acicular apatites, are common in the Laoniushan complex. Compared with the host rocks, they have lower SiO 2 (48.6–53.7 wt.%) and higher Mg # (51–56), Cr (122–393 ppm), and Ni (24–79 ppm), but equivalent Sr–Nd isotope compositions, indicating that the MMEs likely originated from an ancient enriched lithospheric mantle. The abundance of MMEs in the granitoid intrusions suggest that magma mixing plays an important role in the generation of the Laoniushan complex. Collectively, it is suggested that the Laoniushan complex was a product of post-collisional magmatism related to lithospheric extension following slab break-off. Formation of the adakitic and shoshonitic intrusions in the Laoniushan complex indicates that the Qinling Orogen had evolved into a post-collisional setting by about 230–210 Ma.