Fluid generation and evolution during exhumation of deeply subducted UHP continental crust: Petrogenesis of composite granite–quartz veins in the Sulu belt, China

Abstract

Composite granite–quartz veins occur in retrogressed ultrahigh pressure (UHP) eclogite enclosed in gneiss at General's Hill in the central Sulu belt, eastern China. The granite in the veins has a high‐pressure (HP) mineral assemblage of dominantly quartz+phengite+allanite/epidote+garnet that yields pressures of 2.5–2.1 GPa (Si‐in‐phengite barometry) and temperatures of 850–780°C (Ti‐in‐zircon thermometry) at 2.5 GPa (~20°C lower at 2.1 GPa). Zircon overgrowths on inherited cores and new grains of zircon from both components of the composite veins crystallized at c. 221 Ma. This age overlaps the timing of HP retrograde recrystallization dated at 225–215 Ma from multiple localities in the Sulu belt, consistent with the HP conditions retrieved from the granite. The εHf(t) values of new zircon from both components of the composite veins and the Sr–Nd isotope compositions of the granite consistently lie between values for gneiss and eclogite, whereas δ18O values of new zircon are similar in the veins and the crustal rocks. These data are consistent with zircon growth from a blended fluid generated internally within the gneiss and the eclogite, without any ingress of fluid from an external source. However, at the peak metamorphic pressure, which could have reached 7 GPa, the rocks were likely fluid absent. During initial exhumation under UHP conditions, exsolution of H2O from nominally anhydrous minerals generated a grain boundary supercritical fluid in both gneiss and eclogite. As exhumation progressed, the volume of fluid increased allowing it to migrate by diffusing porous flow from grain boundaries into channels and drain from the dominant gneiss through the subordinate eclogite. This produced a blended fluid intermediate in its isotope composition between the two end‐members, as recorded by the composite veins. During exhumation from UHP (coesite) eclogite to HP (quartz) eclogite facies conditions, the supercritical fluid evolved by dissolution of the silicate mineral matrix, becoming increasingly solute‐rich, more ‘granitic’ and more viscous until it became trapped. As crystallization began by diffusive loss of H2O to the host eclogite concomitant with ongoing exhumation of the crust, the trapped supercritical fluid intersected the solvus for the granite–H2O system, allowing phase separation and formation of the composite granite–quartz veins. Subsequently, during the transition from HP eclogite to amphibolite facies conditions, minor phengite breakdown melting is recorded in both the granite and the gneiss by K‐feldspar+plagioclase+biotite aggregates located around phengite and by K‐feldspar veinlets along grain boundaries. Phase equilibria modelling of the granite indicates that this late‐stage melting records P–T conditions towards the end of the exhumation, with the subsolidus assemblage yielding 0.7–1.1 GPa at <670°C. Thus, the composite granite–quartz veins represent a rare example of a natural system recording how the fluid phase evolved during exhumation of continental crust. The successive avail...

On the survival of intergranular coesite inUHPeclogite

“…Thus, at the metamorphic peak there may have been only a limited amount of fluid in isolated pores or remaining along grain boundaries (cf. Ferrando, Frezzotti, Dallai, & Compagnoni, ; Frezzotti & Ferrando, ) or the crust may have been largely fluid absent (Wang et al., ). This is consistent with an expectation that subducting continental crust generally will lose most of its water during subduction (e.g.…”

“…Exsolution of structural water from the NAMs in the isolated F 1 fold hinges was limited, and the low in the coesite eclogite was insufficient to promote the transformation of coesite to quartz. Differential distribution of fluids at a larger scale is demonstrated by a comparison between the outcrops at Yangkou and those just to the south at General's Hill (Figure b), where there was sufficient fluid available that it became locally channelized in fractures, as recorded by composite granite–quartz veins in retrogressed (migmatitic) eclogite (Wang et al., ).…”

“…Nonetheless, an eclogite mineral assemblage and granoblastic microstructures are still preserved in some isoclinally folded layers of residual eclogite. Zircon from General's Hill consistently records positive δ 18 O values that do not match the large‐scale O isotope depletion signature for zircon from other UHP metamorphic rocks exposed in the Sulu belt (Wang et al., ). This difference implies that the protoliths of the UHP rocks at General's Hill, and probably also at Yangkou Bay, were not affected by the Neoproterozoic high‐temperature meteoric–hydrothermal alteration recorded in rocks elsewhere in the belt (e.g.…”

Into the melting pot: A celebration of the career of Michael Brown

“…However, he has continued his work on high-grade metamorphism, with important contributions investigating open system melting and the role of accessory minerals during anatexis (Yakymchuk & Brown, 2014a, 2014b. Latterly, he has extended his interests to high-P and ultrahigh-pressure metamorphism of continental crust Xia, Brown, Wang, Wang, & Piccoli, 2018), in particular with respect to the evolution of fluids and melts under these extreme conditions (Wang et al, 2017). A very recent paper, published with co-author Stephan Sobolev in Nature, extrapolates models of modern-day plate tectonics into the past to suggest that the rise of the continents and an increasing supply of sediments to continental margins post-3 Ga provided the lubrication required to stabilize subduction (Sobolev & Brown, 2019).…”

Partial melting due to breakdown of an epidote‐group mineral during exhumation of ultrahigh‐pressure eclogite: An example from the North‐East Greenland Caledonides

“…Documenting partial melting textures of UHP eclogite on both meso-and micro-scales is critical for understanding anatexis of metabasite. On the outcrop scale, felsic veins, irregular patches, and lenses of granitic (Gilotti, McClelland, & Wooden, 2014;Liu, Robinson, & Liu, 2012;McClelland, Gilotti, Mazdab, & Wooden, 2009;Wang et al, 2017;Zhao, Zheng, Wei, & Wu, 2007), dacitic (Wang et al, 2014), tonalitic (Chen, Liu, Sun, & Zhu, 2010), and trondhjemitic composition (Labrousse, Prouteau, & Ganzhorn, 2011;Shatsky et al, 1999) have been interpreted as crystallized melt derived from eclogite. Geochemical characteristics of the melt composition may aid interpretation of the melting process; for example, the depletion of heavy rare earth elements is a feature of trondhjemite veins in eclogite and is attributed to partial melting in the presence of garnet (Shatsky et al, 1999).…”