Insights into Collisional Magmatism from Isotopic Fingerprints of Melting Reactions

Piston-cylinder experiments in the granite system demonstrate that a wide variety of isotopically distinct melts can arise from progressive melting of a single source. The relation between the isotopic composition of Sr and the stoichiometry, of the observed melting reactions suggests that isotopic signatures of anatectic magmas can be used to infer melting reactions in natural systems. Our results also indicate that distinct episodes of dehydration and fluid-fluxed melting of a single, metapelitic source region may have contributed to the bimodal geochemistry of crustally derived leucogranites of the Himalayan oroge

Rapid Changes in Australian Plate Velocity Due to Collisions in New Guinea and Ontong Java Indicated by High-Resolution 40Ar/39Ar Geochronology of Australian Intraplate Volcanoes

Subduction zones are the most complex tectonic environments on Earth, as exemplified by the dynamic subduction systems in the southwest Pacific that have undergone numerous episodes of collisional orogenesis, slab rollback, initiation of new subduction zones, and subduction polarity reversal during the Cenozoic. One of the major challenges in modern geodynamics is to understand the timing and duration of these events within subduction zones, where the evidence may be variably obscured by difficult access in remote, rugged, vegetated, or submarine terrains, overprinting by subsequent tectonism, or even loss of evidence if rocks are subducted. A powerful approach to determine the age and duration of tectonic events within subduction zones is the record of paleo-plate velocities for the interacting plates; changes in plate velocity are recorded by deviations in the geometry and age progression of plume-derived intraplate volcanic chains. On the Australian plate high-resolution 40Ar/39Ar geochronology of hotspot-derived volcanism has resolved a 3-Ma duration (from 26 to 23 Ma) reduction in plate velocity, attributed to collision with the Ontong Java Plateau (Knesel et al. 2008, Nature vol. 454, p754-757). In this abstract, we report further geochronology of Australian intraplate volcanoes, which demonstrates an earlier episode of reduced plate velocity spanning from 30.5 to 29 Ma. This period of reduced velocity is correlated to collisional orogenesis in New Guinea or New Caledonia, which caused the cessation of convergence along the Papuan-Rennel trench and intensification of subduction along the Manus-Kilinailu-North Solomon system. The brief duration of this episode of reduced plate velocity - spanning just over one million years - underscores the rapidity with which plate boundaries can respond through the cessation of subduction along one system, and the resultant intensification of subduction along another zone

Rapid Changes in Australian Plate Velocity Due to Collisions in New Guinea and Ontong Java Indicated by High-Resolution 40Ar/39Ar Geochronology of Australian Intraplate Volcanoes

Subduction zones are the most complex tectonic environments on Earth, as exemplified by the dynamic subduction systems in the southwest Pacific that have undergone numerous episodes of collisional orogenesis, slab rollback, initiation of new subduction zones, and subduction polarity reversal during the Cenozoic. One of the major challenges in modern geodynamics is to understand the timing and duration of these events within subduction zones, where the evidence may be variably obscured by difficult access in remote, rugged, vegetated, or submarine terrains, overprinting by subsequent tectonism, or even loss of evidence if rocks are subducted. A powerful approach to determine the age and duration of tectonic events within subduction zones is the record of paleo-plate velocities for the interacting plates; changes in plate velocity are recorded by deviations in the geometry and age progression of plume-derived intraplate volcanic chains. On the Australian plate high-resolution 40Ar/39Ar geochronology of hotspot-derived volcanism has resolved a 3-Ma duration (from 26 to 23 Ma) reduction in plate velocity, attributed to collision with the Ontong Java Plateau (Knesel et al. 2008, Nature vol. 454, p754-757). In this abstract, we report further geochronology of Australian intraplate volcanoes, which demonstrates an earlier episode of reduced plate velocity spanning from 30.5 to 29 Ma. This period of reduced velocity is correlated to collisional orogenesis in New Guinea or New Caledonia, which caused the cessation of convergence along the Papuan-Rennel trench and intensification of subduction along the Manus-Kilinailu-North Solomon system. The brief duration of this episode of reduced plate velocity - spanning just over one million years - underscores the rapidity with which plate boundaries can respond through the cessation of subduction along one system, and the resultant intensification of subduction along another zone