Early Cretaceous mafic dikes in the northern Qinling Orogenic Belt, central China: Implications for lithosphere delamination

摘要

Mafic dikes preserve a considerable amount of geological information about crustal evolution. Yanshanian granitoids and granitites are widespread in the southern margin of the North China Craton (NCC) and northern Qinling Orogenic Belt (NQOB), respectively. However, further study is required to resolve the many contrasting hypotheses relating to the mechanisms of petrogenesis and origins. The present study therefore reports on geochemical, geochronological, and Sr-Nd-Pb isotopic data for a suite of mafic dikes in the Mokou area of the NQOB, central China. The Mokou dikes are composed of diabase and lamprophyre intruded into the Kuanping Group (Pt-3). The zircon U-Pb ages of two diabases and one lamprophyre were 132 +/- 3 Ma and 132 +/- 1 Ma, and 132 +/- 1 Ma, respectively. The ages of a total of 126 zircon grains from three rock samples were assigned to nine age groups: similar to 2.2 Ga, similar to 946 Ma, similar to 519 Ma, similar to 440 Ma, similar to 413 Ma, similar to 385 Ma, similar to 160 Ma, similar to 145 Ma, and 132 Ma. This complex zircon age spectrum corresponds to regional magmatic or metamorphic events from the Paleoproterozoic to the Early Cretaceous. We determined that the Mokou dikes are dominantly alkaline rocks that belong to the shoshonite and high-K calc-alkaline series. The Mokou dike samples were enriched in large ion lithophile elements (e.g., Rb and Ba) and presented pronounced negative anomalies in the high-field-strength elements (e.g., Nb, Ta, Zr, and Hf). Both diabase and lamprophyre in the Mokou area were enriched in light rare earth elements (11.9 (La/Yb)(N) 30.8) with no obvious Eu anomalies (delta Eu = 0.92-1.03). They had high initial Sr-87/Sr-86 ratios (0.70625-0.71433), negative epsilon (Nd) (t) values (-12.9 to -8.3), and slightly variable initial ratios of Pb-206/Pb-204 (17.539_18.185), Pb-207/Pb-204 (15.518-15.693), and 99.) Our Pb-208/Pb-204 (38.090_38.996) results indicate that the Mokou dikes originated from an enriched lithospheric mantle and a contaminated asthenosphere. Continental crust materials in the magmas/fluid reservoirs were involved during the rapid intrusion of the Mokou mafic magma, as opposed to the assimilation of wall rocks in the ascent conduit. We conclude that the Mokou dikes are products of lithosphere delamination, which occurred ca. 132 Ma in the NQOB, thus suggesting that lithospheric thinning of the NCC also affected the NQOB.