Magmatic evolution and source of a Proterozoic rapakivi granite complex in the North China Craton: New evidence from zircon U - Pb ages, mineral compositions, and geochemistry

摘要

Typical Proterozoic rapakivi granite of the Northern Hemisphere occur in Precambrian shield areas, and are commonly associated with anorthosites, mangerites, and charnockites, thus belonging to the AMCG (anorthosite-mangerite-charnocidte-granite) suite. A similar suite is represented in the North China Craton (NCC) by the Shachang rapalcivi granite and Damiao anorthosite complexes. The Shachang complex consists of amphibole-biotite rapakivi granite, porphyritic biotite granite, coarse-grained biotite granite, medium-to fine-grained biotite granite, and two-mica granite (listed from the earliest to the latest phases). New zircon U-Pb dating shows that the complex was formed over the period 1700-1682 Ma. The mafic minerals of the complex are calcic amphibole (ferroedenite-ferrohastingsite) and biotite (annite-siderophyllite), and the compositions of the amphibole suggest that the matrix of the amphibole-biotite rapakivi granite crystallized at a pressure of similar to 4 kbar. The temperatures of crystallization, obtained from the compositions of amphibole, biotite, and zircon, decreased from similar to 780 degrees C for the amphibole-biotite rapakivi granite to similar to 660 degrees C for the two-mica granite. Our study of the Shachang complex indicates an evolution from early potassium-rich magmas to late silicon-rich magmas. The early amphibole-biotite rapakivi granite crystallized under relatively high temperatures (760-780 degrees C) and low fO(2), while the later rocks formed under lower temperatures (760-660 degrees C) and high fO(2). The Shachang rapakivi granite and Damiao anorthosite complexes contain different mafic minerals and show diverse trace element ratios and disparate geochemical evolutionary trends, suggesting different sources. Zircon Lu-Hf and whole-rock Sin-Nd isotopic data for the Shachang complex are relatively homogeneous, with epsilon(Hf)(t) and epsilon(Nd)(t) values varying from -9.8 to -3.3 and -6.4 to -3.4, respectively. Corresponding two-stage Hf model ages and Nd depleted mantle model ages vary from 2650 to 2850 Ma and from 2480 to 2840 Ma, respectively. These model ages are similar to the ages of Neoarchean lower crustal metamorphic rocks in the NCC (2500-2800 Ma). In addition, the compositions of the amphiboles and biotites in the granite in the Shachang complex indicate derivation from the crust. All the evidence points to the Shachang rapakivi granite complex having been formed from magmas derived from the lower crust of the NCC, rather than from the fractionation of mantle-derived mafic magmas with minor crustal contamination. This paper provides a detailed case study of the Shachang complex that will further our understanding of the sources of Proterozoic rapakivi granite in the Northern Hemisphere that belong to the AMCG suite of rocks.