Geochemical and Sr–Nd–Pb isotopic compositions of Cretaceous granitoids: constraints on tectonic framework and crustal structure of the Dabieshan ultrahigh-pressure metamorphic belt, China

Abstract

The Dabie orogenic belt can be divided into four tectonic and lithological units. They are, from north to south, the North Huaiyang (NHY) unit, the Northern Dabie complex (NDC) unit, the ultrahigh-pressure (UHP) metamorphic unit, and high-pressure (HP) metamorphic unit. Cretaceous granitoids from the four tectonic units, irrespective of tectonic unit and lithology, show surprisingly similar major and trace element and particularly Sr–Nd–Pb isotopic compositions, indicating a similar crustal source for their magma genesis. They have initial ( 87Sr/ 86Sr) t values of 0.7068–0.7091 and Nd-depleted mantle model ages of 1.6–2.4 Ga and ε Nd( t) values at the time of their emplacement (120 Ma) ranging from −13 to −26. Their present-day Pb isotopic compositions are 206Pb/ 204Pb=15.62–17.06, 207Pb/ 204Pb=15.21–15.44, and 208Pb/ 204Pb=36.59–37.81. With comparison to the Sr–Nd–Pb isotopic compositions of the basement rocks in the Dabie orogenic belt, the isotopic compositions of the Cretaceous granitoids indicate a source that is similar to the Northern Dabie complex but distinct from the ultrahigh-pressure metamorphic unit. This strongly suggests that the exposed Northern Dabie complex extends in the deep crust southward beneath the UHP/HP metamorphic units and northward beneath the North Huaiyang unit. The Northern Dabie complex is best interpreted as the core of a dome within the Dabie orogenic belt. The UHP/HP metamorphic unit is only a thin-skinned slab confined in shallower crustal levels over the Northern Dabie complex. Present tectonic framework for the UHP/HP terrain was formed by extensional tectonics and post the UHP/HP metamorphism. The Northern Dabie complex has an affinity to the Yangtze craton. The similar source for the Cretaceous granitoids also suggests that the suture between the North China and the Yangtze cratons is marked by the Xiaotian–Mozitang Fault separating the North Huaiyang unit and the Northern Dabie complex rather than the previously thought Wuhe–Shuiko Fault, which separates the Northern Dabie complex and the UHP unit.