We have been investigating the structural and metamorphic effects of deep continental subduction in the Sulu orogeny, in particular, focusing on evidence for partial melting of eclogites in subduction zones. This research made a breakthrough in 2014 and was published in Nature Communications (T1). Results show that deeply subducted eclogite in the Mesozoic Sulu orogen were subducted to >120 km depth at 230 Myr ago, then partially melted during their early retrograde path to the surface 228–219 Myr ago. This contribution represents the first documented example of field, petrographic, microstructural, geochemical and geochronological evidence in the world for partial melting of deeply-subducted eclogite.
Geological map of Yangkou bay and General’s Hill. (a) Simplified geological map of the Sulu orogen and its location in China. Scale bar, 100 m at 50-m intervals. (b) Geological map of Mt. Laoshan and the structural setting of Yangkou Bay and General’s Hill. Scale bar, 1 km at 0.5 km intervals. (c) Map of continuously exposed coastal outcrops at General’s Hill. Scale bar, 30 m at 15–m intervals. Our detailed 1:1,500 scale mapping delineates strongly foliated and
complexly folded retrogressed eclogite, cut by channels of dominantly felsic leucosome. The most weakly retrogressed part of the eclogite body consists of strongly foliated isoclinally folded eclogitic gneiss, interlayered with foliated felsic leucosome and retrogressed eclogite (now garnet-bearing amphibolite). In other places the eclogite is preserved as sheared boudins with leucosome and quartz veins in pressure shadows of the eclogitic boudins. Mapping by L. Wang, T. Kusky, S. J. Wang, J. P. Wang and Y. Ding.
A) P–T–t path of the UHP eclogite and eclogitic residue in the Yangkou and General’s Hill area, Sulu Belt. Eclogite from UHP stage-1contains no phengite; however, phengite in eclogite from UHP stage-2 to
quartz eclogite facies rises to 5–15%. These three types of eclogite samples were taken from
Yangkou Bay, eclogitic residue sample is taken from General’s Hill. Our P–T estimation is based on the geobarometer of Waters & Martin and the geothermometers of Ravna and Green & Hellman. AM, amphibolites facies; Amp-EC, amphibole eclogite facies; BS, blueschist facies; Dry-EC, dry eclogite facies;EA, epidote amphibolites facies; GS, greenschist facies; HGR, high- pressure granulite facies; LGR, low-pressure granulite facies; Lws-EC, lawsonite eclogite facies; Zo-EC zoisite eclogite facies.
B) Model showing partial melting of subducted eclogite and how melt channels aid exhumation, feed crustal lavas and make seismic bright spots. The elliptical insets a–d represent the progressive different stages and scales of partial melting of eclogite and melt segregation during exhumation..Eclogite begins partial melting by initial melt droplets forming along grain boundaries (a, scale bar, 50 mm), which then coalesce into 4–10 m wide intergranular veinlets (b, scale bar, 1 cm), which then move along foliation planes and extensional shear zones, eventually forming larger melt pockets in low-stress zones such as fold hinges between units with different rheologies (c, scale bar, 10 cm). Melts in these pockets then merge through interaction of melting and deformation, enhancing deformation in these zones, forming melt channels consisting of 50% melt and 50% residual eclogite (d, scale bar,1 m). Where these melt channels merge melts escape and form metre-scale dikes that may interact with melts derived from the gneisses and transport magma to higher lithospheric levels. SCLM, subcontinental lithospheric mantle; NCC, North China Craton.
This year we will focus on the relationship between the partial melting of the ultrahigh pressure rocks in Sulu orogen and its deformation and metamorphism, and its geodynamic significance. Moreover, we will cooperate with two overseas deputy directors of the center, Professor M. Brown from University of Maryland, and Professor Ali Polat from University of Windsor. Through joint training of doctoral students and international cooperation, some achievements has been made:
in the eclogite which occurred during partial melting, and discussed the melt/liquid environment of the eclogite system during continental deep subduction. The research paper has been accepted by SCI journal American Mineralogists (T2) and will be published in March 2016.
(2) Professor Ali Polat, the deputy director of the overseas of Center for Global Tectonics, and Associate Professor Wang Lu summarized the style of deformation and generation of felsic rocks on outcrop scales in the Archean craton of West Greenland and the Mesozoic Sulu orogenic belt of eastern China, and found that they are similar, suggesting that the mechanism of continental crust formation in the Archean and Phanerozoic is similar. This research paper has been published in Tectonophysics (T2).
(3) Data collection has almost been finished and we have begun to write two papers to provide key evidence about where the melting and fluid came.
from experimental petrology
It is proposed in the subduction channel model that the plate interface interaction is a basic mechanism for the mass and energy exchange between Earth's surface and interior. The significant difference in composition and nature between continental lithosphere and oceanic lithosphere inevitably leads to variations in deep physical and chemical processes as well as crust-mantle interaction products in these two settings. Many studies of experimental petrology have provided constraints on the potential partial melting and crust-mantle interaction in oceanic subduction channels for silicate and carbonate rocks. The partial melts of mafic and felsic compositions are adakitic or non-adakitic granitic melts depending on melting pressure or depth. A trivial amount of CO2 can lower significantly the melting temperature of peridotites and lead to pronounced enrichment of incompatible elements in carbonate melt. The silica saturated or unsaturated melts can react with mantle-wedge peridotites in subduction channels to generate complex products. However, the existing experiments are mostly dedicated to island arc settings above oceanic subduction zones rather than dehydration melting above continental subduction zones. It is crucial to conduct high pressure and high temperature experiments to investigate all possible reactions between peridotites and crustal materials and their derivatives under the conditions responsible for the slab-mantle interface in continental subduction channels. Experimental results, combined with natural observations, are possible to elucidate the processes of metamorphic dehydration, partial melting and mantle metasomatism in continental subduction channels. These research results have been published in Science China Earth Sciences (1 paper, T3).
Contact: Junfeng Zhang
Schematic diagrams of oceanic and continental subduction channels (after Guillot et al. (2009) and Zheng (2012)). (a) The oceanic subduction channels are thin (<1 km to >10 km) and
characterized by high temperature, low viscosity, high water fugacity and syn-subduction dehydration partial melting. (b) The continental subduction channels are thick (>5 km to >30 km) and characterized by low temperature, high viscosity, low water fugacity and syn-exhumation or post-collisional magmatism.
Crust-mantle interaction in the continental subduction-collision zone
In the past years, we mainly focused on research about crust-mantle interaction along continental subduction zones, especially about deeply subducted continental crust and we choose the Late Triassic alkaline complex in the Sulu UHP terrane as an example. Our studies suggest a crust-mantle interaction along the continental subduction interface as follows: (1) hydrous felsic melts from partial melting of subducted continental crust during its exhumation and metasomatized the overlying mantle wedge to form a K-rich and amphibole-bearing mantle; (2) partial melting of the enriched lithospheric mantle generated the Late Triassic alkaline complex under a post-collisional setting; and (3) the alkaline magma experienced subsequent fractional crystallization mainly dominated by olivine, clinopyroxene, plagioclase and alkali feldspar. These research results have been published in Gondwana Research (1 paper, T2).
Contact: Haijin Xu
Sr and Nd isotopic compositions of the Shidao alkaline complex. Initial Sr and Nd isotopic ratios were calculated at
210Ma. Data sources: the referenced Shidao alkaline rocks are from Gao et al. (2004), Yang et al. (2005), Chen and Jiang (2011) and Zhao et al. (2012); eclogites and granitic gneisses in the Dabie–Sulu UHP terrane are from Jahn (1998), Li et al. (2000), Ma et al. (2000), Tang et al. (2008), Xu et al. (2008) and Song et al. (2014a); and mantle wedge peridorites in Dabie–Sulu HUP terrane
are from Yang and Jahn (2000), Zhang et al. (2000)and references therein.