Late Permian to Early Triassic mafic to felsic intrusive rocks from North Liaoning, North China: Petrogenesis and implications for Phanerozoic continental crustal growth

摘要

Zircon U-Pb dating, whole-rock major oxide, trace element and Sr-Nd-Hf isotopic data are presented for the Late Permian to Early Triassic Shijianfang batholith from Faku in North Liaoning Province, North China. Two main magmatic suites are documented: one, with an emplacement age of ca. 260 Ma, is mafic to intermediate in composition, forming small gabbroic to monzonitic intrusions; the other is felsic and quartz monzonitic to granitic in composition, with an emplacement age of ca. 250 Ma. Geochemically, the rocks from the mafic suite show strong enrichment of LILE and LREE and depletion in HFSE, and also have moderately depleted isotopic compositions, with I_(Sr) ranging from 0.7049 to 0.7054, ε_(ND)(t) from +2.72 to - 1.82 and zircon ε_(Hf)(t) values from +3.4 to +7.1. These features suggest derivation from high degree partial melting of a subduction-related metasomatized lithospheric mantle source. The rocks from the felsic suite range from 66.7 to 77.2 wt% SiO2, and define a continuous chemical evolutionary trend, from less-differentiated low-silica members displaying LREE enriched patterns, no Eu anomalies and high Ba, Sr, Zr and Hf, to more-differentiated, high-silica members with higher alkalis, strong depletion in Ba, Sr and Ti, enrichment in HFSE, fractionated REE patterns and strong negative Eu anomalies. However, they exhibit indistinguishable isotopic compositions from the mafic rocks, with I_(Sr) ranging from 0.7043 to 0.7062, ε_(Nd)(t) from + 2.28 to - 0.55 and zircon ε_(Hf)(t) values from + 3.6 to + 6.6. This suggests that the parental magma for the felsic suite originated from partial melting of mixed protoliths composed of juvenile basaltic underplate and ancient lower crustal materials. Subsequent fractional crystallization, with overprinting by magmatic hydrothermal fluids, can explain the geochemical variations of the felsic suite.The juvenile character of both lithospheric mantle and crustal rocks, as recorded by the Shijianfang batholith, suggests that the northern Liaoning block forms part of a Phanerozoic accretionary orogenic belt This observation indicates that the Chifeng-Kaiyuan Fault likely represents the Mesozoic lithospheric boundary between the North China Craton and the Xing-Meng Orogenic Belt in northern Liaoning. The Shijianfang batholith, together with Late Permian to Middle Triassic adakitic and A-type rocks, mafic-ultramafic cumulates and alkaline intrusions from the neighboring areas, constitutes an important post-collisional to intra-plate anorogenic magmatic province within the continental interior of the newly amalgamated North China-Mongolian plate. The unique mafic and felsic coupling in the Shijianfang batholith provides a good proxy record for the multiple-step vertical continental crustal growth in the newly amalgamated continental interior during the Phanerozoic, triggered by continued magmatic underplating due to lithospheric delamination and hot asthenospheric upwelling within a post-collisional to post-orogenic extensional regime.