TY - JOUR

T1 - Importance of melt fraction and source rock composition in crustal genesis - the example of two granitic suites of northern Portugal

AU - Holtz, François

N1 - Copyright: Copyright 2014 Elsevier B.V., All rights reserved.

PY - 1989/12

Y1 - 1989/12

N2 - In the Montalegre area (northern Portugal) two varieties of syn- to late-tectonic Hercynian aluminous granitoids are distinguished: 1. (1) Muscovite-rich, two-mica granites, forming many successively emplaced bodies with little chemical variation. The chemical characteristics of these units suggest partial melting of various crustal rocks under distinct P-T and water saturation conditions. There has been no homogenization between the different melt batches. The rocks contain very few restitic minerals. Segregation processes may have operated either in the source rock or in an early magmatic stage. 2. (2) A cordierite-, biotite- and muscovite-bearing granitic body. Its mineralogical composition is extremely heterogeneous and it contains many xenoliths. The genesis of this granite results essentially from partial melting of peraluminous orthogneisses. The melt fraction was higher than the rheological critical melt percentage (RCMP). During anatexis, the rigid framework of the source rock broke down, leading to migration of both melt and residual minerals towards upper-crustal levels. The heterogeneity of the resulting granite is mostly related to an incomplete separation of residual crystals and melt. The existence of biotite- and cordierite-rich samples is the result of accumulation of residual minerals during the magma emplacement. This study emphasizes the role of the source rock composition, which controls the degree of partial melting at given P-T, and water saturation conditions during crustal melting. The strong geochemical differences between the two kinds of granitoids are related to the near absence of residual minerals in the muscovite-rich granites and to the presence of residual minerals in the cordierite-bearing granite. It is suggested that the muscovite-rich granites are mostly low-melt-fraction ( < RCMP) granites. In contrast, the source rock of the cordierite granite underwent partial melting with a high melt fraction ( > RCMP). The presence of many residual crystals (alkali feldspars and plagioclases as well as dark minerals) in the cordierite granite implies a high viscosity of the melts and therefore water-undersaturated melting conditions.

AB - In the Montalegre area (northern Portugal) two varieties of syn- to late-tectonic Hercynian aluminous granitoids are distinguished: 1. (1) Muscovite-rich, two-mica granites, forming many successively emplaced bodies with little chemical variation. The chemical characteristics of these units suggest partial melting of various crustal rocks under distinct P-T and water saturation conditions. There has been no homogenization between the different melt batches. The rocks contain very few restitic minerals. Segregation processes may have operated either in the source rock or in an early magmatic stage. 2. (2) A cordierite-, biotite- and muscovite-bearing granitic body. Its mineralogical composition is extremely heterogeneous and it contains many xenoliths. The genesis of this granite results essentially from partial melting of peraluminous orthogneisses. The melt fraction was higher than the rheological critical melt percentage (RCMP). During anatexis, the rigid framework of the source rock broke down, leading to migration of both melt and residual minerals towards upper-crustal levels. The heterogeneity of the resulting granite is mostly related to an incomplete separation of residual crystals and melt. The existence of biotite- and cordierite-rich samples is the result of accumulation of residual minerals during the magma emplacement. This study emphasizes the role of the source rock composition, which controls the degree of partial melting at given P-T, and water saturation conditions during crustal melting. The strong geochemical differences between the two kinds of granitoids are related to the near absence of residual minerals in the muscovite-rich granites and to the presence of residual minerals in the cordierite-bearing granite. It is suggested that the muscovite-rich granites are mostly low-melt-fraction ( < RCMP) granites. In contrast, the source rock of the cordierite granite underwent partial melting with a high melt fraction ( > RCMP). The presence of many residual crystals (alkali feldspars and plagioclases as well as dark minerals) in the cordierite granite implies a high viscosity of the melts and therefore water-undersaturated melting conditions.

UR - http://www.scopus.com/inward/record.url?scp=0024885376&partnerID=8YFLogxK

U2 - 10.1016/0024-4937(89)90013-3

DO - 10.1016/0024-4937(89)90013-3

M3 - Article

AN - SCOPUS:0024885376

VL - 24

SP - 21

EP - 35

JO - Lithos

JF - Lithos

SN - 0024-4937

IS - 1

ER -