Modeling metamorphism in collisional orogens intruded by magmas: II. Fluid flow and implications for Barrovian and Buchan metamorphism, Scotland

Abstract

This paper presents the results of two-dimensional numerical modeling of crustal fluid flow in a collisional overthrust setting during emplacement of syn-metamorphic mafic sills. An important feature of the fluid flow in the presence of magmatic intrusions is the long-lived prograde up-temperature (up-T) flow of fluid. This upward flow is driven by dehydration reactions in the rocks underlying the intrusions and upward is the consequence of the inverted thermal profile that appears within the depth range of magmatism. If magma is intruded at mid-crustal depths following crustal collision, the up-T flow regime may persist for as much as 5 Myr. Such long-lived up-T fluid flow is not observed in models without magmatism. Strong retrograde reactions in rocks surrounding intrusions may give rise to downward up-temperature flow into the heated but rapidly cooling areas of the crust during exhumation, but the fluid fluxes in these areas are typically small. The total time-integrated fluid fluxes produced by magmatism and crustal overthickening in the middle and deep crust are substantial, in some cases reaching 3,000 m3 m−2 over 35 Myr. Early magmatic events may give rise to fluid fluxes exceeding 1,000 m3 m−2 in only 1 Myr. The strong dehydration that accompanies the emplacement of hot magma into the relatively low-temperature wall rock leads to hydrofracturing of large volumes of crustal rocks surrounding intrusions. The fractured rock region may extend over tens of kilometers from the zone of magma emplacement. More hydrofracturing is predicted for the middle and deep crust than for the shallow crust because of the exponential decrease of crustal permeability with depth that is assumed in our models. Endothermic dehydration, exothermic retrograde hydration, and the latent heat of wall rock fusion can all influence thermal budgets. In the final part of the paper we integrate the results of the fluid flow modeling presented here, the reconstructions of thermal evolution presented in Part I, and field-based data for the type locality of Barrovian and Buchan metamorphism in Scotland. Many key phenomena predicted by our numerical models are observed in the Scottish Dalradian, including the very short thermal peaks attained over a relatively narrow time span across a broad range of metamorphic zones; penecontemporaneous Barrovian, Buchan, and granulite facies metamorphism; steep Metamorphic Field Temperature Gradients (MFTGs) in the vicinity of intrusions; and petrological evidence consistent with up-temperature fluid flow. Our results suggest that some controversial aspects of regional metamorphism in the Scottish Dalradian, including heat transfer processes and the steep MFTGs, may be plausibly explained by the effects of plutonism.