Precambrian tectonic evolution
1.1 Main research contents and progress
1.1.1 Precambrian tectonic evolution of North China Craton
The North China craton consists of the Eastern Block, Western Block, and the intervening Central Orogenic belt which contains at least two sutures. Significant progress was made on the easternmost circa 2.5 Ga Zanhuang-Zunhua suture, focused mainly on the structural relationships of different geological units, ages of plutons intruded into mélange associated with opening and closing of an Archean ocean along this belt, and petrogenesis of the Archean intrusive rocks that cut the mélange. We proposed that the Eastern Block and an arc terrane (Fuping terrane) in the COB collided prior to 2.5 Ga, forming the extensive mélange belt. A subduction polarity reversal event followed the arc-continent collision, causing additional deformation and forming the intrusive rocks that cut across the mélange and parts of the Eastern Block. The ocean that remained open behind the Fuping arc closed sometime between 2.4 and 2.1 Ga, causing additional deformation and amalgamating the Eastern and Western Blocks. From 1.9-1.85 Ga the craton experienced a craton wide granulite facies event. We relate this to a continent-continent collision on the north margin of the craton, probably related to when the North China Craton collided with the Columbia (Nuna) Supercontinent. These research results have been published in Lithos (2 papers, T2). We also initiated research on the Dengfeng greenstone belt and proposed that the sedimentary-volcanic rocks in the greenstone belt may represent a Neoarchean subduction-accretionary complex correlative with the Zanhuang Complex. The research results have been published in Precambrian Research (1 paper, T2).
Contact: Wang Junpeng, Deng Hao
Model of interpreted geodynamic origin of the ca. 2.5 Ga magmatic event in the Central Orogenic Belt of the NCC. A new east-dipping reversed subduction happened at 2.5 Ga, following the collision between the Eastern Block of the NCC and the intra-oceanic arc terrane to the west. This east-dipping subduction polarity reversal event results in the melting of the enriched mantle, and then gives rise to the formation of the ca. 2.5 Ga mafic dikes and granites (including the Wangjiazhuang granite) of the Eastern Block.
In addition, Kusky and Mooney (U.S. Geological Survey) studied the geological and geophysical characters of Ordos Block and found S-wave anomalies at different depths, in common with the Sichuan and Tarim Basins that are also underlain by cratonic crust. Seismic velocity sections show that the Ordos Block has thick crust, with the Moho deeper than 40 km. It is underlain by a denser lower crustal body with seismic velocities of 7.1 km/s which is characteristic of oceanic plateaus globally. Comparisons were made directly with the southern Caspian Sea, another trapped oceanic plateau in an orogenic belt, and the Wrangellia terrane, an oceanic plateau accreted to western North America. We proposed an hypothesis that the Ordos Basin had an origin as an oceanic plateau that accreted to the NCC, and later experienced different episodes of differentiation associated with later subduction and collisions. The formation of cratonic lithosphere by accretion of oceanic plateaus may be one mechanism to create stable cratons. These research results have been published in Earth and Planetary Science Letters (1 paper, T1).
Contact: Tim Kusky, Walter Mooney
Map of free-air gravity anomalies in SE Asia (modified from Yang and Liang, 2011).
Topography and free-air gravity maps of the southern Caspian Sea and surrounding regions. Data to generate the topographic map is from the National Geophysical Data Center (NGDC; http:/ /www.ngdc .noaa .gov/) and the free air anomaly was calculated via the International Centre for Global Earth Models (ICGEM; http:/ /icgem .gfz-potsdam .de /ICGEM/ICGEM .html). Both maps were drawn using GMT (Generic Mapping Tools) software.
S-wave anomalies at different depths (modified from Bao et al., 2015). The Ordos Craton has a clear high-S-wave anomaly, in common with the Sichuan and Tarim Basins that are also underlain by cratonic crust. Panels a, b, c and d correspond to the depths upper crust (panel a), mid-lower crust (panel b), Moho-100 km (panel c), and
100–150 km below surface (panel d).
1.1.2 Precambrian tectonic evolution of the Yangtze Craton and Cathaysia Orogen
We documented Neoproterozoic (0.94-0.93 Ga) metamorphic events for the first time in the Miaowan ophiolite on the northern margin of the Yangtze craton, which provides important evidence for the final amalgamation of the basement of the Yangtze craton. We identified two stages of metabasite- ultrabasic rocks in the Miaowan ophiolitic complex; latest Mesoproterozoic (1.12-1.1 Ga) and Neoproterozoic (1.0-0.97 Ga), formed in different tectonic settings. In Cenxi, southeast Guangxi, early Paleozoic ophiolite remnants and high-Mg basaltic andesite and andesite has been recognized, which provides crucial evidence for the existence of Paleozoic ocean crust subduction-accretionary and collision-orogenesis in the Cathaysian terranes.
We studied the Archean-Paleoproterozoic high-grade metamorphic rocks in the core of the Huangling dome, the northern margin of Yangtze craton through detailed geological mapping, geochronology and geochemical methods. We found that the Paleoproterozoic mafic- ultramafic rocks in the volcano-sedimentary sequence formed in an arc environment above a subduction zone and we proposed that records of a Paleoproterozoic subduction-collision event are preserved in the Huangling dome on the northern margin of Yangtze craton. This research paper is about to be submitted to an international SCI journal (T2).
In addition, we describe the field relationships on the basis of geological mapping, then present detailed and systematic geochemical, geochronology and Sm-Nd isotopic investigations on these mafic-ultramafic rocks in the Jiangnan orogenic belt (JOB) between the Yangtze craton and Cathaysian terranes in South China. Geochemical signatures suggest that those rocks formed in an extensional arc environment, and the subduction may have continued to ca. 750 Ma in the western JOB, implying that the amalgamation event between the Yangtze craton and Cathaysian terranes was later than 750 Ma. This research has been published in Gondwana Research (T2).
Contact: Peng Songbai
Map of the Yangtze craton showing areas discussed.
The presence or absence of a thick lithospheric root beneath the Yangtze Craton has been debated. We integrated and analyzed existing geological, geophysical and geochemical data for the first time and the results indicate that the Yangtze Craton has lost the eastern half of its root, much like the North China craton. The processes of destruction can be divided into Neo- proterozoic and Mesozoic stages. The major decratonization occurred in the Mesozoic, which is closely related to the subduction and rolling back of the Pacific Plate. Results have been published in Tectonophysics(T2).
After comprehensive and comparative analysis of the Yilgarn Craton, Superior Craton and NCC, it has been proposed that the lithospheric thinning at the edges of cratons is a common phenomenon, related to the effects of orogenic events on cratonic margins. In addition, a comparison of the NCC with cratons that have lost parts of their roots via impingement by mantle plumes (Tanzania, North Atlantic) shows that plumes can interact with the Mid- lithospheric discontinuity, aiding in large-scale foundering of SCLM. A review paper on this is in review. Other related results have been published in Canadian Journal of Earth Sciences (T4).
Contact: Tim Kusky, Xiaoyong Li, Zhengshen Wang
(a) Simplified tectonic map of the NCC and surrounding orogenic belts (modified from Kusky and Li 2003, Zhai et al. 2010, and Kusky 2011). Note that the craton is divided into two main blocks (Eastern and Western) by the COB. Note that known Archean granulites are confined to the COB, but that the ca. 1.9–1.8 Ga granulites are concentrated in a belt along the northern margin of the craton south of the North Hebei Orogenic Belt, and on the Shandong Peninsula. (b) Simplified tectonic map of the Slave craton (drawn after Hoffman 1989b) and its relationships to surrounding orogens and basins. Note that the Slave craton is much smaller than the NCC. HP, high pressure; HT, high temperature; UHT, ultra-high temperature; GSLsz, Great Slave Lake shear zone.
along two different vertical profiles (a, b) and the distribution of the mantle transition thickness of the Yangtze and North China cratons (c, d). (a), (b) are P wave velocity perturbations profiles along 37°N and 31°N latitude, respectively (Modified from Huang and Zhao, 2006, with permissions); (c) Distribution of the mantle transition thickness of the NCC (Chen and Ai, 2009, with
permissions); (d) Distribution of the mantle transition thickness of the Yangtze craton (Modified from Huang et al., 2014a, with permissions). The red in a and b represents the slow velocities while the blue represents the fast velocities. The fast velocities outline the flat-lying slabs beneath Eastern Asia. The areas with blue in c and d are characterized by thick mantle transition zone (N250 km) while the areas with red are characterized by thin mantle transition zone (b250 km).
New comprehensive model for craton destruction through flat slab dehydration, slab rollback, mantle influx, melt-generation, and melt SCLM peridotite reaction.
1.2 Scientific research projects
We have three National Natural Science Fund Projects (41572203; 41172069; 41372075)
this year about Precambrian tectonic evolution.
1.3 Research papers
We have published 1 paper in T1 journal, 5 papers in T2 journals, 1 paper in T3 journal and
3 papers in T4 journals during 2014-2015 about this research direction.