Crustal Growth in Continental Collision Zones and Crustal “Destruction” in Continental Interiors by Granitoid Magmatism

The standard island arc model has served the community well for understanding continental crust accretion because island arc andesites satisfy the andesitic composition of the bulk continental crust and because both continental crust and island arc rocks share a common geochemical “arc signature” characterized by relative enrichment in fluid-soluble elements (e.g., U, K, Pb) and depletion in fluid-insoluble elements (e.g., Nb, Ta, Ti). However, rigorous analysis of geological, petrological and geochemical observations can readily reveal fundamental shortcomings of the island arc model, which, among others, include: (1) primary arc magmas are not andesitic, but basaltic; (2) magmatic crust production at arcs is mass-balanced by subduction erosion and sediment recycling with no net crust accretion; (3) arc magmatic rocks are overall more depleted in incompatible elements than needed for continental crust; (4) island arc magmatism cannot explain the episodic continental crust growth over earth’s history. Our collaborative studies on the ~55 ± 5 Ma syncollisional granitoids associated with the India-Asia collision in southern Tibet (Mo et al. 2007, 2008, 2009) led to the hypothesis that continental collision zones are primary sites of net continental crustal growth (Niu et al. 2013). Our test on syncollisional granitoids in older orogens in the vast Tibetan Plateau and the adjacent regions (i.e., the ~220 Ma West Kunlun, ~240 Ma East Kunlun, ~440 Ma Qilian and ~220 Ma West Qinling orogens) fully support this hypothesis. We also test the hypothesis that the Cretaceous granitoids along the southeast coastline of continental China mark the end of paleo-Pacific subduction because of the trench jam caused by the arrival and collision of the Chinese continental shelf (the basement of East and South China Seas) of exotic origin at ~100 Ma (Niu et al. 2015). The latter may be an oceanic plateau, but is more likely a continental mass probably affiliated with western Australia in its history that is to be further tested. Our study of the Jurassic-Cretaceous granitoids in the interiors of continental China supports the hypothesis that these granitoids represent crustal response to the lithosphere thinning as the result of basal hydration weakening with the water coming from the paleo-Pacific slab stagnant in the mantel transition zone (Niu et al. 2015). Accompanying the widespread lithosphere thinning in the vast region of eastern continental China is the generation of basaltic magmas whose intrusion and underplating caused the deep crustal anatexis and widespread granitoids randomly distributed throughout eastern China in time (~190–88 Ma with a peak at ~140 Ma) and space (in a wide E-W zone in excess of 1000 km). These granitoids of deep crustal origin contribute no new crustal accretion, but “destruct” the existing lower crust with the mafic granulite residues becoming the sub-Moho constituents, which resolves the puzzle (Gao et al. 1998a, b) why eastern China has thinner and less mafic lower crust that is also detected seismically (Li et al. 2006). Granitoid magmatism can thus both contribute to juvenile crustal accretion and destruct the existing crust in response to varying aspects of plate tectonics.