Crystallization coupled with gravity removal of depleted interstitial melt has long been recognized as a mechanism of magma differentiation. Similarly, heat released by synplutonic basaltic magma intrusions has long been recognized as capable of driving convection in granite chambers. Direct evidence of these processes has seldom been described in granites. In the Tavares pluton, we mapped a number of melt extraction structures from pores of a crystal-liquid mush (an effectively solid magma where crystals form an interconnected skeleton) and a variety of flow structures such as (1) meter-scale tear-or mushroom-shaped blobs representing within-chamber diapirs; (2) meter-scale ellipsoids representing frozen thermal plumes of granite, driven by heat released from disrupted diorite intrusions; and (3) ladder dikes and snail structures representing cross sections of several superposed cylindrical magma channels (possibly feeders of diapirs and plume heads). A fundamental feature of the structures in the Tavares pluton is that they are well delineated by mafic schlieren developed at active channel margins. We postulate a new model for the origin of marginal schlieren, which combines shear flow sorting and melt escape from the flowing magma into an effectively solid surrounding mush. Extraction structures (representing melt

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Recent studies argue that subduction-modified, Cu-fertilized lithosphere controls the formation of porphyry Cu deposits in orogenic belts. However, it is unclear if and how this fertilization process operates at cratonic edges, where numerous large non-arc Au-rich deposits form. Here we report data from lower crustal amphibolite and garnet amphibolite xenoliths hosted by Cenozoic stocks that are genetically related to the Beiya Au-rich porphyry deposits along the western margin of the Yangtze craton, China. These xenoliths are thought to represent cumulates or residuals of Neoproterozoic arc magmas ponding at the base of arc at the edge of the craton that subsequently underwent high-pressure metamorphism ca. 738 Ma. The amphibolite xenoliths are enriched in Cu (383-445 ppm) and Au (7-12 ppb), and a few garnet amphibolite xenoliths contain higher Au (6-16 ppb) with higher Au/Cu ratios (2 × 10 −4 to 8 × 10 −4 ) than normal continental crust. These data suggest that metal fertilization of the base of an old arc at the edge of the craton occurred in the Neoproterozoic via subduction modification, and has since been preserved. The whole-rock geochemical and zircon Hf isotopic data indicate that melting of the Neoproterozoic Cu-Au-fertilized low-crustal cumulates at 40-30 Ma provided the metal endowment for the Au-rich porphyry system at the cratonic edge. We therefore suggest that the reactivated cratonic edges, triggered by upwelling of asthenosphere, have the potential to host significant Au ore-forming systems, especially non-arc Au-rich porphyry deposits. LOWER CRUSTAL XENOLITHS AND THEIR ORIGINAbundant xenoliths have been found with Eocene stocks and associated volcanic rocks at six locations exposed along the cratonic edge (Fig. 1B). Amphibolites and garnet amphibolites are the primary types, the former hosted by Liuhe syenites and Beiya monzogranite porphyries ( Fig. 2A), and the latter widely occurring in the Liuhe stock (Fig. 2B). Their mineralogical and whole-rock compositions, and zircon, oxide, and sulfide geochemical *

The recent discovery of large porphyry copper deposits (PCDs) associated with Miocene (22-12 Ma) granitoid magmas in the eastern section of the Paleocene-Eocene Gangdese magmatic arc in the Himalaya-Tibetan orogenic belt raises new questions about the origin of water-rich (≥4.5 wt.%), oxidized (ΔFMQ 1-3) magmas in continental collisional settings and their mineralization potential. We review the literature and compile available data on whole rock and isotope geochemistry for Cenozoic igneous rocks from Tibet, and add new zircon Ce4+/Ce3+ and Ti-in-zircon thermometry data to better understand variations in oxidation state and thermal evolution of these suites, which are key controls on Cu mineralization. Six distinct Cenozoic igneous suites are defined: Paleocene-Eocene syn-collisional Gangdese magmatic arc rocks (ΔFMQ =-1.2 to +0.8) (suite I), and five broadly contemporaneous Miocene suites. A distinct change in magmatism along the length of the belt occurs at around 88°E in the Miocene suites: to the east, porphyry copper mineralization is associated with a moderately oxidized, high-Sr/Y granitoid suite (suite II, ΔFMQ = +0.8 to +2.9) with minor occurrences of transitional (hybrid) monzonitic (suite III) and trachytic rocks (suite IV; both with zircon Ce4+/Ce3+ > 50-100, EuN/EuN* =~0.5, and ΔFMQ =~+1 to +2). To the west of 88°E, trachytic volcanic rocks (suite V) are more voluminous but more reduced (zircon Ce4+/Ce3+ < 50, ΔFMQ <+1), and are associated with sparse, poorly mineralized high-Sr/Y granitoids (suite VI) which are moderately oxidized (zircon Ce4+/Ce3+ = 20-100, ΔFMQ =~+1 to +3). The Miocene high-Sr/Y granitoids have many compositional and isotopic similarities to the Paleocene-Eocene Gangdese arc rocks, and are interpreted to have been derived by melting of the hydrated arc root, with minor mantle input. In contrast, the highly evolved isotopic signatures of the Miocene trachytic rocks, combined with deep seismic profiles and a xenolith-derived geotherm, suggest their derivation from the underthrust Indian Proterozoic subcontinental lithospheric mantle (SCLM) or old fore-arc Tibetan SCLM during phlogopite breakdown at temperatures of~1100°C. Based on published geophysical data and tectonic reconstructions, we develop a model that explains the origin of the various Miocene magmatic suites, their spatial differences, and the origin of related PCDs. Following the early stages of continental collision (Eocene-Oligocene), shallow underthrusting of the Indian continental lithosphere and subcretion of Tethyan sediments (including oxidized carbonates and possibly evaporites) under eclogite facies conditions promoted the release of aqueous fluids, which hydrated and oxidized the base of the overlying Tibetan plate. This metasomatism rendered the Tibetan lower crust fusible and fertile for metal remobilization. During the mid-Miocene, the Indian slab steepened in the eastern sector (east of~88°E). In this eastern belt, deeply derived trachytic magmas were trapped in melt zones at the base of the Tibetan crust, ...

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The secular evolution of the Earth's crust is marked by a profound change in average crustal chemistry between 3.2 and 2.5 Ga. A key marker for this change is the transition from Archaean sodic granitoid intrusions of the tonalite–trondhjemite–granodiorite (TTG) series to potassic (K) granitic suites, akin (but not identical) to I-type granites that today are associated with subduction zones. It remains poorly constrained as to how and why this change was initiated and if it holds clues about the geodynamic transition from a pre-plate tectonic mode, often referred to as stagnant lid, to mobile plate tectonics. Here, we combine a series of proposed mechanisms for Archaean crustal geodynamics in a single model to explain the observed change in granitoid chemistry. Numeric modelling indicates that upper mantle convection drives crustal flow and subsidence, leading to profound diversity in lithospheric thickness with thin versus thick proto-plates. When convecting asthenospheric mantle interacts with lower lithosphere, scattered crustal drips are created. Under increasing P-T conditions, partial melting of hydrated meta-basalt within these drips produces felsic melts that intrude the overlying crust to form TTG. Dome structures, in which these melts can be preserved, are a positive diapiric expression of these negative drips. Transitional TTG with elevated K mark a second evolutionary stage, and are blends of subsided and remelted older TTG forming K-rich melts and new TTG melts. Ascending TTG-derived melts from asymmetric drips interact with the asthenospheric mantle to form hot, high-Mg sanukitoid. These melts are small in volume, predominantly underplated, and their heat triggered melting of lower crustal successions to form higher-K granites. Importantly, this evolution operates as a disseminated process in space and time over hundreds of millions of years (greater than 200 Ma) in all cratons. This focused ageing of the crust implies that compiled geochemical data can only broadly reflect geodynamic changes on a global or even craton-wide scale. The observed change in crustal chemistry does mark the lead up to but not the initiation of modern-style subduction.This article is part of a discussion meeting issue ‘Earth dynamics and the development of plate tectonics’.

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