Abstract
Pressure–temperature–time paths obtained from minerals in metamorphic rocks allow the reconstruction of the geodynamic evolution of mountain ranges under the assumption that rock pressure is lithostatic. This lithostatic pressure paradigm enables converting the metamorphic pressure directly into the rock’s burial depth and, hence, quantifying the rock’s burial and exhumation history. In the coherent Monte Rosa tectonic unit, Western Alps, considerably different metamorphic pressures are determined in adjacent rocks. Here we show with field and microstructural observations, phase petrology and geochemistry that these pressure differences cannot be explained by tectonic mixing, retrogression of high-pressure minerals, or lack of equilibration of mineral assemblages. We propose that the determined pressure difference of 0.8 ± 0.3 GPa is due to deviation from lithostatic pressure. We show with two analytical solutions for compression- and reaction-induced stress in mechanically heterogeneous rock that such pressure differences are mechanically feasible, supporting our interpretation of significant outcrop-scale pressure gradients.
Highlights
Pressure–temperature–time paths obtained from minerals in metamorphic rocks allow the reconstruction of the geodynamic evolution of mountain ranges under the assumption that rock pressure is lithostatic
Lithostatic pressure is commonly applied as a paradigm for the reconstruction of the burial and exhumation history of rock units, such that the maximum pressure estimate for a coherent rock unit is converted into its burial depth
For the range of compositions measured, this high XOH content reflects the H2O activity. This supports the assumption of H2O activity of ca. 1 and the 2.2 GPa estimated for the whiteschist
Summary
Pressure–temperature–time paths obtained from minerals in metamorphic rocks allow the reconstruction of the geodynamic evolution of mountain ranges under the assumption that rock pressure is lithostatic. The studied field area is part of the prominent Monte Rosa nappe, an internal Penninic unit of the Western Alps It belongs to the Briançonnais domain and represents the most distal European continental crust involved in the Alpine orogeny[4] (Fig. 1a). The whiteschists are not xenoliths or pieces of a subduction channel that could have been incorporated later by tectonic mixing (mélange) into the metagranite during the Alpine orogeny (see below) Instead, they were formed by metasomatic alteration of the granite by late magmatic hydrothermal fluids related to the cooling of the granite intrusion long before the onset of Alpine orogenesis[8]. The granitic protolith of the whiteschist and the late magmatic, pre-Alpine, hydrothermal nature of the fluids are confirmed by the following: field observations of a gradual transition over few metres from metagranite to whiteschist, marked by the progressive disappearance of igneous phases, demonstrating the in-situ character of the whiteschist (Supplementary Fig. 1); similarity in chemical composition shown by mass balance calculations using whole rock compositions, confirming the granitic origin of the whiteschist[8]
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