Metamorphic evolution of pelitic rocks of the Monte Rosa nappe: Constraints from petrology and single grain monazite age data
Creators: name: Engi, Martin
name: Scherrer, Nadim
name: Burri, Thomas
Publisher: name: (:unav)
Year: 2001
Subjects: Monte Rosa Alps polymetamorphism metapelites monazite dating exhumation high pressure high-pressure metamorphism lago-di-cignana western alps alpine metamorphism rb-sr garnet zone geochronology geobarometry assemblage

description: A suite of metapelites from the pre-granitic basement of the Monte Rosa nappe was collected and investigated to gain insight into the evolution of this important upper Penninic thrust sheet. Careful sampling at mesoscopic to microscopic scale allowed the identification of relics of distinctly pre-Alpine origin, of assemblages formed in the early Alpine high pressure history of the nappe, and of features due to the Meso-Alpine thermal overprint. Variscan structural relics, e.g. large monazite grains with relic cores, or monazite inclusions within garnet porphyroblasts, were recognized in the field and at thin section scale. Alpine features include high pressure and younger thermal phases; these are not readily separable in the field, but they are usually distinctly identifiable at thin section scale. Combined chemical Th-U-Pb dating of monazite by electron microprobe and novel XRF-microprobe analysis, verified and enhanced by isotopic laser ablation PIMMS analysis, was performed on single monazite grains selected in thin section, with the full context information preserved. Based on electron microprobe data for assemblages from these same thin sections, TWQ thermobarometry indicates metamorphic conditions preserved along the P-T-t path of a polyorogenic evolution. In only two samples monazite inclusions in old garnet were found to record ages of 330 Ma, corresponding to the intrusion of the main Mote Rosa granodiorite mass. High-grade Permian metapelites appear variably affected - in isolated cases even unaffected - by all Alpine activity, retaining mineral assemblages, consistent P-T-information, and monazite grains (even within the sample matrix) formed some 270 Ma ago in response to the intrusion of granitic masses in the Monte Rosa nappe. This same stage of low to medium pressure metamorphism was detected in monazite inclusions in garnet from numerous samples taken at various localities. Reaching sillimanite+Kspar grade, this stage is evident in migmatic restites as well and is suggested to represent either a regional event, at 10-20 km depth, associated with the generation and emplacement of an extensive suite of acid dykes and small stocks; alternatively, and more or less equivalently, one may think of this phase as indicating widespread contact metamorphism. No evidence whatsoever could be found supporting a questionable Cretaceous high-pressure phase. Consistent TWQ pressures between 9.2 +/- 1.8 and 12 +/- 1.5 kbar, with temperatures ranging from 595 +/- 25 to 755 +/- 65 degreesC, represent the dominant Alpine overprint documented in samples containing assemblages kyanite + garnet + phengite +/- staurolite. Median conditions are 11.0 +/- 1.2 kbar and 652 +/- 41 degreesC, with no significant regional gradient, Monazite single grain dates range from 46 to 31 Ma, but with most of the accurate ages lying between 38 and 32 Ma. Several petrological observations suggest that this stage probably represents (partial) re-equilibration upon decompression, rather than the maximum depth reached by the Monte Rosa nappe. Late Alpine thermal overprinting is evidenced by only very few monazite ages of &lt;30 Ma obtained, and no precise P-T-brackets could be obtained for that retrograde stage. Recent palcogeographic models for the Alpine evolution derive the Monte Rosa nappe from the southern European continental margin. Our results demand that the subduction system implicated this nappe sufficiently early to yield peak metamorphic temperatures near 700 C at 35-30 km depth in the Eocene, probably during extrusion following subduction and slab breakoff. Higher pressure relics are known from several areas along the margin of the Monte Rosa nappe, including the UHP-fragments of Lago Cignana. These relics appear to be restricted to the immediate periphery of the Monte Rosa nappe, outlining a discontinuous mantle of highly strained fragments of ophiolitic and metasedimentary trails. Whether the Furgg Zone, at least its northern parts, may also represent a tectonic melange, is currently controversial. It seems likely that strain concentration in marginal zones of the Monte Rosa thrust sheet enabled the tectonic emplacement of the several massive granitic sheets that make up the bulk of the nappe. These were left largely intact, thus preserving pre-Alpine and early Alpine assemblages in metapelites protected within less deformed portions of the metagranitoids.