Partial melting of deeply subducted eclogite from the Sulu orogen in China

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Abstract

We report partial melting of an ultrahigh pressure eclogite in the Mesozoic Sulu orogen, China. Eclogitic migmatite shows successive stages of initial intragranular and grain boundary melt droplets, which grow into a three-dimensional interconnected intergranular network, then segregate and accumulate in pressure shadow areas and then merge to form melt channels and dikes that transport magma to higher in the lithosphere. Here we show, using zircon U-Pb dating and petrological analyses, that partial melting occurred at 228-219 Myr ago, shortly after peak metamorphism at 230 Myr ago. The melts and residues are complimentarily enriched and depleted in light rare earth element (LREE) compared with the original rock. Partial melting of deeply subducted eclogite is an important process in determining the rheological structure and mechanical behaviour of subducted lithosphere and its rapid exhumation, controlling the flow of deep lithospheric material, and for generation of melts from the upper mantle, potentially contributing to arc magmatism and growth of continental crust.

Figures

  • Figure 1 | 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 km at 50-km intervals. (b) Geological map of Mt. Laoshan and the structural setting of Yangkou Bay and General’s Hill35. Scale bar, 1 km at 0.5 km intervals. (c) Map of continuously exposed coastal outcrops at General’s Hill. Scale bar, 30m at 15–m intervals. Our detailed 1:1,500 scale mapping delineates
  • Figure 2 | Structural map of melt channels at General’s Hill. Multiple leucosome veins, melt pockets and melt channels merging to form dikes at General’s Hill are shown within the map (see Fig. 1c for location, noting the difference in orientation of maps). Most eclogite is retrogressed into garnetbearing amphibolite deformed into rootless isoclinal and less-common sheath folds, and disaggregated into boudins surrounded by leucosome. Their strong foliation is mostly defined by biotite and amphibole. These folds typically have thicker hinges than limbs (ptygmatic folds) or are strongly sheared and boudinaged along their limbs. In some locations, the hinges of the isoclinal folds are also sheared, thinned and broken into boudins with felsic leucosome flowing into the boudin necks and pressure shadows behind fold hinges of layers with stronger competence than surrounding layers. Once the melt was present in these regions, the melt enhanced the deformation, further localizing strain and melt concentration in these locations. The melts appear as a leucocratic matrix and flows around the retrogressed eclogite layers. The axial planes are almost coincident with the NW-striking foliation that dips steeply to the NE. 7E, geochronological sample location of melted eclogite as shown in Fig. 7c (stage II). Stage III, location of last stage of melting process as shown in this figure. Mapping by T. Kusky, L. Wang, S. J. Wang, J. P. Wang and Y. Ding. Original scale 1:2,000; scale bar, 5m at 1-m intervals. Base map provided by Laoshan National Park.
  • Figure 3 | Sketches and associated photographs showing the partial melting processes. (a) Stage-I. Early stage of partial melting, finger-shaped leucosome starts to aggregate within the hinge of the isoclinal eclogite fold. Scale bar, 5 cm. Sketches or field photo pointed to the blue and red box represent enlarged area where the boxes are. Field photo connected to the red box by the black arrow shows the irregular boundary between leucosome and residue that supports a magmatic partial melting genesis, scale bar in this photo is 1 cm across. (b) Stage-II. Medium stage of partial melting, melt channels (leucosome) interlayered and flowing surrounding the sheared folded eclogite. Scale bar, 10 cm. Red dashed box represents the same range of the field photo on its right column. (c) Stage-III. Mature stage of partial melting, melt aggregates into larger veins and then forms felsic dikes. Scale bar, 100 cm. The red box represents the same spatial range as the field sketch.
  • Figure 4 | Microphotographs of partially melted eclogites and leucosome (melt) in the Sulu orogen. (a–d) YK05-2a, UHP stage-1 eclogite (GrtþOmpþCoe/Qz, Ph free) from Yangkou. (a) Intergranular coesite with higher relief and surrounded by retrogressed quartz with lower relief. (b,c) Rounded interstitial pod composed of KfsþQz with fractures cross-cutting and filled with Kfs. (d) MS inclusion composed of PhþBt and KfsþAbþBrtþ Ep in garnet with veinlet and inclusion of Phþ Bt connected, indicating in situ partial melted phengite inclusions within garnet and omphacite grains. (e) YK12-3a, phengite-bearing Qz-eclogite (GrtþOmpþQzþ Ph) from Yangkou, phengite in situ dehydration melting, forming a MS inclusion of Kfsþ PlþBa-Kfs within garnet, connecting with veinlets filled by BtþKfsþ Pl next to the phengite. (f) YK12-3a, MS inclusion of KfsþQz within omphacite with radial fractures. (g) YK128-15, eclogitic residue with mineral assemblage of GrtþOmpþQz from General Hill; sym represents symplectite replacing previous omphacite. MS inclusion of KfsþQz developed within garnet, surrounded by Hbþ PlþMgt rim. (h,i) YK128-15, cuspate veinlets of Kfsþ Plþ Ep (melt droplets) with low dihedral angles, and form ‘strings of beads’ along grain boundaries between quartz and at triple junctions. (j) 09PMS-1A, leucosome sample from Fig. 1d, inset photo of stage I, clear triple junction texture developed at grain boundaries of quartz grains, with K-feldspar filling in the triple junction. (k,l) YK128-16a, leucosome sample, more advanced stages of partial melting where the melt droplets (Kfsþ Pl) along grain boundaries have merged and formed an interconnected 3D network along grain boundaries and micro-cracks enabling the melt to drain out of the intergranular areas into low-stress regions in the leucosome. Scale bars, 15mm in e,f and 50mm in a–d and g–l. a and j are microscope photographs under cross-polarization, the rest are all back-scattered electron images. Ab, albite; Ap, apatite; Bt, Biotite; Coe, coesite; Grt, garnet; Hb, hornblende; Kfs,
  • Figure 5 | Geochemical plots of ten rock pairs of leucosome and residue. These samples were collected in the region between two quartz porphyry dikes of Fig.1c. (a) Geochemical plot of Yangkou Bay leucosome and residue samples in the Nb/Y versus Zr/TiO2 ratio diagram from 58. (b) Distribution of Yangkou Bay leucosome samples in the FeOtotal-Na2OþK2O-MgO ternary diagram from ref. 59, wt.%, weight percent. (c) Diagram for Sr/Y ratios versus Y contents (p.p.m.) for the Yangkou Bay leucosomes. The fields for adakites and typical arc rocks are after Defant and Drummond51. See original data from Supplementary Data Set 1 and 2.
  • Figure 6 | P–T–t path of the UHP eclogite and eclogitic residue in the Yangkou and General’s Hill, Sulu Belt. Ages of prograde, peak
  • Figure 7 | Concordia diagrams and REE plots. (a) Concordia diagram and weighted average age of one leucosome sample with representative zircon CL images. (b) Concordia diagram and weighted average age of one residue sample with representative zircon CL image. (c) Concordia diagram and weighted average age of one mixture sample contains both leucocratic and residual material (melting process stage II) with representative zircon CL image.
  • Figure 8 | 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, 50mm), which then coalesce into 4–10m wide

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Wang, L., Kusky, T. M., Polat, A., Wang, S., Jiang, X., Zong, K., … Fu, J. (2014). Partial melting of deeply subducted eclogite from the Sulu orogen in China. Nature Communications, 5. https://doi.org/10.1038/ncomms6604

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