Mercury isotope evidence for the importance of recycled fluids in collisional ore systems.

The sources of fluids and metals in porphyry systems of continental-collision settings are poorly constrained. Mercury isotopes display unique mass-independent fractionation (expressed as Δ199Hg) and may provide important constraints on metal and volatile sources given that Hg is a highly volatile metal. Here, we report Hg isotope data on ore-forming porphyries, barren magmatic rocks, and mantle-derived mafic magmas from southern Tibet. The fertile porphyries and coeval mafic magmas display mainly positive Δ199Hg values (up to +0.25 per mil), while Δ199Hg values in barren magmatic rocks and mafic magmas are largely negative (-0.54 to 0.00 per mil). The positive Δ199Hg values observed here are consistent with seawater and marine sediments, suggesting that the ultimate source of fluids involved in the genesis of post-subduction porphyry copper deposits was the mantle lithosphere metasomatized by previous oceanic plate subduction. Our Hg isotope data provide an alternative view to current metallogenetic models on collisional porphyry systems that focus on melting of the lower continental crust.

Seismically imaged lithospheric delamination and its controls on the Mesozoic Magmatic Province in South China.

The current lithospheric root of the South China Block has been partly removed, yet what mechanisms modified the lithospheric structure remain highly controversial. Here we use a new joint seismic inversion algorithm to image tabular high-velocity anomalies at depths of ~90-150 km in the asthenosphere beneath the convergent belt between the Yangtze and Cathaysia blocks that remain weakly connected with the stable Yangtze lithosphere. Based on obtained seismic images and available geochemical data, we interpret these detached fast anomalies as partially destabilized lower lithosphere that initially delaminated at 180-170 Ma and has relaminated to their original position after warming up in the mantle by now. We conclude that delamination is the most plausible mechanism for the lithospheric modification and the formation of a Mesozoic Basin and Range-style magmatic province in South China by triggering adiabatic upwelling of the asthenosphere and consequent lithospheric extension and extensive melting of the overlying crust.

Interplay between oceanic subduction and continental collision in building continental crust.

Generation of continental crust in collision zones reflect the interplay between oceanic subduction and continental collision. The Gangdese continental crust in southern Tibet developed during subduction of the Neo-Tethyan oceanic slab in the Mesozoic prior to reworking during the India-Asia collision in the Cenozoic. Here we show that continental arc magmatism started with fractional crystallization to form cumulates and associated medium-K calc-alkaline suites. This was followed by a period commencing at ~70 Ma dominated by remelting of pre-existing lower crust, producing more potassic compositions. The increased importance of remelting coincides with an acceleration in the convergence rate between India and Asia leading to higher basaltic flow into the Asian lithosphere, followed by convergence deceleration due to slab breakoff, enabling high heat flow and melting of the base of the arc. This two-stage process of accumulation and remelting leads to the chemical maturation of juvenile continental crust in collision zones, strengthening crustal stratification.

Felsic volcanism as a factor driving the end-Permian mass extinction.

The Siberian Traps large igneous province (STLIP) is commonly invoked as the primary driver of global environmental changes that triggered the end-Permian mass extinction (EPME). Here, we explore the contributions of coeval felsic volcanism to end-Permian environmental changes. We report evidence of extreme Cu enrichment in the EPME interval in South China. The enrichment is associated with an increase in the light Cu isotope, melt inclusions rich in copper and sulfides, and Hg concentration spikes. The Cu and Hg elemental and isotopic signatures can be linked to S-rich vapor produced by felsic volcanism. We use these previously unknown geochemical data to estimate volcanic SO2 injections and argue that this volcanism would have produced several degrees of rapid cooling before or coincident with the more protracted global warming. Large-scale eruptions near the South China block synchronous with the EPME strengthen the case that the STLIP may not have been the sole trigger.