Nature and P-T Conditions of the Crust Beneath the Central Mexican Volcanic Belt Based on a Precambrian Crustal Xenolith

摘要

A granulite xenolith was encountered in an ignimbrite of Amealco caldera, Mexico. This sample is one of three granulite occurrences in the central Mexican Volcanic Belt. It is a medium-grained, equigranular rock, with plagioclase (60 vol percent), quartz (10 percent), orthopyroxene (8 percent), clinopyroxene (2 percent), accessory ilmenite (<1 percent), apatite and zircon (both < 1 percent), and glass (20 percent). The glass represents partial fusion, probably due to decompression. Absence of alumina-rich phases or graphite indicates an igneous protolith. Isotope values of ~(143)Nd/~(144)Nd = 0.512681 +- 16 (epsilon Nd = +0.84) and ~(87)Sr/~(86)Sr = 0.705874 +- 39 confirm a volcanic arc tectonic setting. The granulite yielded a model age of 683 Ma. Whole-rock chemistry indicates a dacite composition. REE patterns show a positive Eu anomaly with a general negative slope from La to Lu, and HFSE such as Nb are relatively depleted, as expected for subduction-generated magmas. Fluid inclusion (FI) studies performed mainly on feldspars revealed four FI types. Type 1 is represented by very rare Fl composed of CO_2 +- water. Other types (2, 3, and 4) are essentially composed of CO_2. Type 2 comprises large (up to 60 mu m) FI, whereas type 3 FI are distributed along planes crosscutting the crystal (quartz and feldspar). Type 4 are complex low-maturity FI surrounding type 3 FI. Densities in type 3 FI are between 0.07 and 0.75 g/cm~3. This scattering is interpreted as being due to decrepitation of FI during the decompression stage of the sample. Using the highest recorded densities, maximum trapping pressure was estimated at 2.9 to 3.2 kbar, assuming a temperature of 800-900 deg C, corresponding to a lower to upper crust pressure (5-10 km depth) following the decompression stage. A granulitic basement of arc affinity and Precambrian age is inferred at depths of >5 km beneath Amealco caldera. A fragment of this basement was incorporated into an ascending pulse of mafic magma that was injected into the relatively shallow magma chamber of Amealco caldera, and was erupted together with voluminous pyroclastic flows.