Browsing by Subject "Petrogenesis"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Diffusion, closure temperatures, and accessory mineral petrogeneses in a high-temperature aureole: refining the integration of P-T and t(2003) McFarlane, Christopher R. M.; Connelly, James N.; Carlson, William D.Static heating during intrusion of the 1322 Ma Makhavinekh Lake Pluton (MLP) caused replacement of garnet in the adjacent country rocks (Tasiuyak Gneiss) by coronal assemblages of orthopyroxene + cordierite. Thermometry based on Al solubility in orthopyroxene, applied to relict garnet and neighboring orthopyroxene, preserves a temperature gradient from 700°C to 900°C at distances between 5750 and 20 m from the intrusion, reaffirming the robustness of this thermometry technique. Intracrystalline and intergranular Al zoning in M2 orthopyroxene are interpreted with the aid of numerical models for conductive heat flow in the aureole. The results document the ability of Al-in-orthopyroxene thermometry to preserve a detailed record of thermal histories in contactmetamorphic granulites. Zircon in the Tasiuyak Gneiss formed at ~1850 Ma near the peak of the Torngat Orogeny (M1 metamorphism). These M1 zircons were reheated during contact heating at 1322 Ma (M2 metamorphism). U-Pb dating of M1 zircon, using thermal ionization mass spectrometry (TIMS) and in situ sensitive high resolution ion microprobe (SHRIMP) geochronology, tested whether temperatures in the aureole exceeded the closure temperature for Pb diffusion in zircon. TIMS analyses failed to resolve appreciable resetting towards 1322 Ma even in samples that reached ~900 ºC. In contrast, high spatial resolution SHRIMP data revealed that cores of M1 zircon in samples that reached temperatures >800 ºC are significantly younger than rims. This reversal of core and rim ages implies that Pb was preferentially lost from cores. This observation is hypothesized to be the result of Pb migration during high-temperature recovery of lattice strain induced by higher impurity concentrations (REE, P) in cores. While M1 zircon suffered intracrystalline Pb diffusion during contact heating, it did not react with the surrounding major mineral assemblage. In contrast, low-Y monazite inclusions in M1 garnet were consumed as they were exposed to the M2 coronas. New growth of high-Y monazite records both the age and temperature of metamorphism in the aureole. Dissolution and new growth of monazite occurred under anhydrous conditions, highlighting the potential use of monazite as a high-temperature thermochronometer in granulites and ultra-hightemperature rocks.Item Evidence for upward growth of a layered pluton in the Bindal Batholith, north-central Norway(2007-12) McCulloch, Lindy; Barnes, Calvin G.; Ridley, Moira K.; Yoshinobu, Aaron S.The Hortavær Igneous Complex, off the coast of north-central Norway, is a zoned pluton that exhibits evidence of upward growth as a series of stacked layers or sheets. What crops out now is a pluton with, from west to east, heterogeneous zones of syenite, intense sheeting, diorite, monzonite, and alkali granite. Sheets dip steeply west and strike N-S in the south and N30E in the north, defining the limbs of a regional fold. The pluton intruded calc-silicate, carbonate, and migmatitic gneissic host rocks about 466 Ma. Assimilation and fractional crystallization of dioritic magmas produced syenitic and monzonitic magmas, which are interpreted to have migrated laterally due to outward flow of successive injections of magma. Continued injection of dioritic magma produced more syenite and monzonite. New pulses of diorite spread laterally over variably crystal-rich syenitic magmas. The density contrast between syenitic and dioritic magmas caused syenite to escape upward, producing tubes and flames of syenite that intruded into the overlying sheets. Flame structures in the sheets point to the east, the original top of the pluton. At their bases, dioritic sheets quenched against the syenite and were penetrated by the escape structures. Along their upper surfaces, the dioritic sheets mingled with overlying syenitic magmas to produce enclaves. Magmatic foliation is oriented parallel to the sheets and is defined by alignment of elongate minerals and flattened enclaves. This foliation is interpreted to result from overburden pressure. Folding of the pluton produced parasitic folds of the sheets in the limbs of the regional fold. Subsequent block rotation about 90 degrees east resulted in the current orientation of sheets with a steep westward dip.