Geology, distribution and geochemistry of impact melt at the Mistastin Lake impact crater, Labrador.

摘要

The Mistastin Lake crater in Labrador, Canada (55°53'N 63°18'W) contains a 3 km wide central uplift within a 19 x 12 km wide lake and has a rim diameter of28 km. The projectile impacted Mesoproterozoic crystalline target rocks approximately 36 Ma ago.Melt rocks that are distributed around two thirds of the lake in patchy outcrops vary in thickness from 1m to 80 m. Previous estimates suggested that a coherent melt sheet up 10 200m thick formed in the crater and that the much smaller preserved unit thicknesses are the result of glacial erosion. New field observations and laboratory measurements identify a relationship between distribution, thickness and vesicularity of melt rock units. The thickest melt-rock occurrence, at Discovery Hill, is massive, crystalline, non-vesicular and 80 m thick. In contrast, 1-2 m thick melt-rock occurrences elsewhere in the crater are glassy and vesicular. Measured vesicularities vary from 0.1 to 31% and follow an empirical relationship (&phiv = 30+/-2 h-0.8+/-0.1) whereby vesicularity &phiv increases with decreasing melt rock thickness h. Plagioclase microlite crystallization temperatures of thin melt rock outcrops are very high (> 1300°C), indicating rapid cooling rates. Lower crystallization temperatures (&sim1245°C) for the Discovery Hill melt are consistent with slower cooling rates. The data suggest that the pre-erosional melt sheet at Mistastin was not uniform and, consequently, previous estimates for the level of erosion and the volume of the melt produced have been overestimated.Target rocks which contributed to the impact melt consist principally of anorthosite, mangerite and granodiorite. Chemical compositions of bulk samples of thirty-three melt rocks and fourteen target rocks were measured by XRF and SN-ICPMS. Matrix compositions of nine samples of impact melt rocks were determined by EPMA and LA-ICPMS. Zircon grains from four samples of target rock and zircon clasts from three samples of impact melt rock were measured for multi-element composition, U-Pb age and Hf-isotopic composition by LA-(MC)-ICPMS.The data reveal compositional heterogeneities in the impact melts on the scales of both bulk samples and matrices. Bulk samples can be divided into compositions with high and low concentrations of high-field strength elements (HFSE Ti, Zr, Nb) and Fe, Ba, Ce and Y. High HFSE-type melt rocks formed when impact melt entrained large quantities of clasts from mangerite, which is rich in HFSE. Matrix compositions of bulk samples do not show the HFSE distinction but are affected by the introduction of low-temperature melts from the clasts to form dispersed, micron-scale silica-rich heterogeneities. Both clast entrainment and melting are more extensive for the thicker flow units which had a higher heat capacity for melting and cooled more slowly than thinner flows.This study consists of detailed field observations geology, geochemistry, and geochronology of impact melt and target rocks of the Mistastin impact crater. To determine (1) the significance of the relationship between preserved melt thickness and vesicularity in the melt rocks (2) the scale and origin of compositional heterogeneities in impact melts produced in craters of moderate size and the relationship between entrained mineral clasts and impact melt composition and (3) the origin of zircon clasts in the impact melts.The best estimate of the sources of the initial impact melt is &sim73% anorthosite, &sim7% mangerite and &sim20% granodiorite, based on least-squares modeling of major element compositions of the matrices of thinner flows. Zircon derived from anorthosite can be distinguished from zircon from mangerite and granodiorite on the basis of higher Nb/Ta and Eu/Eu* ratios and more negative initial epsilon Hf values. Zircon clasts greater than 40 microns in size in the impact melt rocks are dominantly or exclusively derived from mangerite and granodiorite. Hence zircon may be a poor provenance indicator for target rock contributors to impact melts.