Geochemistry and geochronology of plutons from the Siziwangqi area, the northern margin of the North China Craton and their implications

Abstract

The evolution of the Central Asian Orogenic Belt (CAOB) is commonly thought to be related to the birth and death of the Paleo-Asian Ocean (PAO), thus the CAOB represents the Paleo-Asian Tectonic Realm. Controversy has long surrounded the polarity of subduction and final closure of the Paleo-Asian Ocean in East Asia during Palaeozoic time. This project attempts to address the issues of whether or not the Paleo-Asian Ocean had experienced southward subduction beneath the North China Craton (NCC) before its closure and when did it start and how long it lasted, if such subduction did exist. In order to constrain these issues, we have carried out extensive field-based petrological, geochemical, and geochronological investigations on the Siziwangqi intrusions of central Inner Mongolia, at the northern margin of the the craton’s Yinshan Block. By the fieldwork, I recognized three major intrusions in the Siziwangqi region. They are the Delinggou intrusion, the Dashuji Intrusion, and the Shibao Intrusion. The Delinggou intrusion was marked as early Paleozoic diorite on the traditional Chinese geological map, and is dominated by diorite and quartz diorite, with minor gabbros and dioritic porphyry. Generally, this rock association is very common in magmatic arcs. The results of this studies revealed that the Paleo-Asian Ocean underwent southward subduction beneath the NCC, and the subduction initiated in the Carboniferous occurred at 343–330 Ma, but the final ocean closure time is not yet constrained. Due to the zircon U-Pb dating revealing that the “Early Paleozoic granitoids” in the traditional 1:200,000 Chinese geological map was formed at ~2.5 Ga, hence it is a further extension of this PhD research project regarding the crustal growth and evolution of the NCC during Neoarchean and Paleoproterozoic times. The granitoids comprise a tonalite-trondhjemite-granodiorite (TTG) association (termed as the Dashuji Intrusion) and alkali feldspar granite (termed as the Shibao Intrusion). Another important achievement I have made during this PhD study is recognition of Neoarchean to Paleoproterozoic granitoids from those “late Paleozoic igneous rocks” marked in the 1:200,000 Chinese geological map, and discovery of the main crustal growth in the Siziwangqi area occurred in the late Neoarchean and involved ~2.5 Ga crustal accretion and minor ~3.0 Ga crustal components. Furthermore, an Andean-type magmatic arc model is proposed to illustrate the late Neoarchean subduction-related arc magmatism within the study area. Our model highlights the NCC experienced a tectonic regime that changed from compressional to extensional during the early Paleoproterozoic time (~2.4Ga), following the ca. 2.5Ga cratonization event, providing an example of granitoid magmatism similar to that was generated under modern plate tectonic regimes.