Petrogenesis of the Mesozoic intrusive rocks in the Tongling area, Anhui Province, China and their constraint on geodynamic process

Abstract

Petrology, element and isotopic geochemistry of the Mesozoic intrusive rocks in the Tongling area were systematically investigated in this study. The intrusive rocks can be divided into two groups, one contains shoshonitic rocks with SiO 2 ≤55%, the other consists mainly of high-potassic calc-alkaline rocks with SiO 2 >55%. The shoshonitic rocks (SiO 2 ≤55%) were generated by the fractional crystallization of the primary basaltic magma sourced from an enriched mantle, then the evolved basaltic magma likely experienced low-degree contamination with the lower crust materials when they ascended. On the other hand, although the intrusive rocks with SiO 2 >55% show most elemental geochemical characteristics similar to an adakite, such as high Na 2 O, Al 2 O 3 , Sr contents, high Sr/Y and La/Yb ratios, they have isotopic compositions much different from an adakite, such as low e Nd(t) (-9.16—16.55) and high ( 87 Sr/ 86 Sr) i (0.7068—0.7105), and some of them show relatively high Y and Yb contents than those of an adakite. We propose that the intrusive rocks with SiO 2 >55% were most probably produced by mixing of the mantle-derived basaltic magma and adakite-like magma derived from the melting of basaltic lower crust that was heated by the underplating mantle-derived shoshonitic magmas. The delamination of lower crust likely took place after or during the formation of these adakite-like rocks in the Tongling area.