Stephen M. Wickham

The segregation and emplacement of granitic magmas

The segregation of granitic magma from residual crystals at low melt-fraction is strongly dependent on the viscosity of the melt. Theoretical considerations imply that for the typical range of granitic melt viscosities (104Pas to 1011Pas) only very limited separation will be possible by a compaction mechanism over the typical duration of a crustal melting event (c. 106 years). Small-scale segregations (millimetre to metre) of the type observed in migmatite terranes may be generated by compaction (possibly assisted by continuous deformation), or by flow of melt into extensional fractures, but low melt-fraction liquids are unlikely to be extracted to form large (kilometre-size) granitic plutons because of the limited separation efficiency. At higher melt-fractions (>30%) the rapid decrease in strength and effective viscosity during partial melting allows other segregation processes to operate. Calculations and experiments indicate that in granitic systems the effective viscosity of partially melted rocks, having a very narrow melt fraction range of 30–50% will fall rapidly to levels at which convective overturn of kilometre-thick zones can occur. Convective motion within anatectic regions is capable of generating large (kilometre-size) homogeneous, high crystal-fraction, crustally-derived magma bodies, which are orders of magnitude greater in size than low melt-fraction segregates. Before convective instability is reached, small (centimetre- to metre-sized) pods of granitic liquid may rise buoyantly through, and pond at the top of such partly molten zones; such a process is consistent with the observation that some granulites appear to be residue rocks, chemically depleted in a minimum melt component. The effective viscosity (and hence the susceptibility to convection) of a partially melted zone within the crust, is strongly dependent on the water content of the system at a given pressure and temperature, because this controls both the quantity of melt generated and also the viscosity of the melt. The intrinsic water content of most crustal lithologies is incapable of promoting the high percentages of partial melting, or the low liquid viscosities, required to form large kilometre-sized granitic plutons by convective homogenization, at typical crustal temperatures. This suggests that the anatexis involved in the generation of large crustally-derived magma bodies has in many cases been promoted by an influx of externally derived aqueous fluid. These magma segregation processes are illustrated with respect to the petrogenesis of three different types of granitoid pluton from a Hercynian low-pressure, metamorphic-anatectic terrane in the Pyrenees.

Stable isotopic evidence for large-scale seawater infiltration in a regional metamorphic terrane; the Trois Seigneurs Massif, Pyrenees, France

Oxygen isotopic analyses of 95 metamorphic and igneous rocks and minerals from a Hercynian metamorphic sequence in the Trois Seigneurs Massif, Pyrenees, France, indicate that all lithologies at higher metamorphic grades than the “andalusite in” isograd have relatively homogeneous δ18O values. The extent of homogenization is shown by the similarity of δ18O values in metacarbonates, metapelites and granitic rocks (+11 to +13), and by the narrow range of oxygen isotopic composition shown by quartz from these lithologies. These values contrast with the δ18O values of metapelites of lower metamorphic grade (δ18O about +15). Homogenization was caused by a pervasive influx of hydrous fluid. Mass-balance calculations imply that the fluid influx was so large that its source was probably high-level groundwaters or connate formation water. Hydrogen isotopic analyses of muscovite from various lithologies are uniform and exceptionally heavy at δD=−25 to −30, suggesting a seawater origin. Many lines of petrological evidence from the area independently suggest that metamorphism and anatexis of pelitic metasediment occurred at depths of 6–12 km in the presence of this water-rich fluid, the composition of which was externally buffered. Deep penetration of surface waters in such environments has been hitherto unrecognized, and may be a key factor in promoting major anatexis of the continental crust at shallow depth. Three types of granitoid are exposed in the area. The leucogranites and the biotite granite-quartz diorite are both mainly derived from fusion of local Paleozoic pelitic metasediment, because all these rocks have similar whole-rock δ18O values (+11 to +13). The post-metamorphic biotite granodiorite has a distinctly different δ18O (+9.5 to +10.0) and was probably derived from a deeper level in the crust. Rare mafic xenoliths within the deeper parts of the biotite granite-quartz diorite also have different δ18O (+8.0 to +8.5) and possibly represent input of mantle derived magma, which may have provided a heat source for the metamorphism.

Stable isotope constraints on the origin and depth of penetration of hydrothermal fluids associated with Hercynian regional metamorphism and crustal anatexis in the Pyrenees

Late Carboniferous (Hercynian) tectonism in the Pyrenees generated extremely steep thermal gradients at 8–14 km depth in the continental crust, producing andalusite- and sillimanite-grade metamorphism and partial melting of Lower Paleozoic metasediments under water-rich conditions. At the same time, amphibolite- and granulitefacies “basal gneisses” were equilibrated under dryer conditions at pressures of 4 to 7 kbar (14–25 km depth), beneath these higher-level rocks. We present 95 new oxygen isotopic analyses of samples from the Agly, St. Barthelemy, Castillon and Trois Seigneurs Massifs, highlighting contrasting 18O/16O systematics at different structural levels in the Hercynian crust, here termed Zones 1, 2, and 3. The unmetamorphosed, fossiliferous, Paleozoic shales and carbonates of Zone 1 have typical sedimentary δ18O values, mostly in the range +14 to +16 for the pelitic rocks and +20 to +25 for the carbonates. The metamorphosed equivalents of these rocks in Zone 2 all have strikingly uniform and much lower δ18O values; the metapelites mostly have δ18O=+10 to +12, and interlayered metacarbonates from the Trois Seigneurs Massif have δ18O of about +12 to +14. Typically, the Zone 3 “basal gneisses” are isotopically heterogeneous with variable δ18O values ranging from +6 in mafic lithologies to +22 in carbonate-rich lithologies. Steep gradients in δ18O (as much as 10 per mil over a few cm) are preserved at the margins of some metacarbonate layers. These data indicate that the Zone 3 gneisses were infiltrated by much smaller volumes of metamorphic pore fluids than were the overlying Zone 2 rocks, and that circulation of surface-derived H2O (either seawater or formation waters, as evidenced by high δD values) was mainly confined to the Paleozoic supracrustal sedimentary pile. This is compatible with an overall reduction of interconnected porosity with increasing depth, but perhaps even more important, the extensive partial melting at the base of Zone 2 may have produced a ductile, impermeable barrier to downward fluid penetration.

Low-pressure regional metamorphism in the Pyrenees and its implications for the thermal evolution of rifted continental crust

During late Palaeozoic (Hercynian) low-pressure regional metamorphism in the Pyrenees, exceptionally high thermal gradients existed within the upper crust, and temperatures as high as 700 °C were attained at depths as shallow as 10 km, resulting in large-scale crustal anatexis. Stable isotope studies indicate that the crust was flushed by circulating ground waters to depths of 12 km, but the amount of fluid involved below 8 km was probably not much greater than 50% of the rock mass, and this fluid apparently did not penetrate the pre-Palaeozoic basement below 12 km. There is no evidence for continental collision in the region at that time, and these data, together with other geological and geophysical constraints, suggest that the most plausible tectonic setting for the metamorphism is a zone of continental rifting, possibly associated with strike-slip movement. Thermal modelling suggests that a transient, high-temperature heat source in the lower crust is required to account for the observed metamorphic P- T arrays. Among a range of possible solutions, a basaltic sill, 6-8 km thick and emplaced at 14 km could generate a maximum temperature array similar to those observed in the Pyrenees.

A strontium, neodymium and oxygen isotope study of hydrothermal metamorphism and crustal anatexis in the Trois Seigneurs Massif, Pyrenees, France

Nd, Sr, and O isotope analyses have been made on metamorphic and igneous rocks and minerals from a 310–340 Ma Hercynian-age metamorphic terrane in the Pyrenees, France. Lower Paleozoic shales and phyllites have 87Sr/86Sr values of 0.707–0.717 at 310 Ma, but model values at 310 Ma of 0.709–0.736 (based on assumed depositional age of 450 Ma and an initial 87Sr/86Sr=0.707). On a regional scale, 87Sr/86Sr was homogenized to about 0.713 to 0.717 in the higher-grade pelitic schists during metamorphism. Much of this 87Sr/86Sr exchange occurred at very low grades (below the biotite isograd), but significant changes also accompanied the δ18O lowering of the phyllites (+13 to +16) during their transformation to andalusite- and sillimanite-grade schists (δ18O=+11 to +12); all of these effects are attributed to pervasive interactions with hydrothermal fluids (Wickham and Taylor 1985). The data also show that a syn-metamorphic plutonic complex, dominated by a biotite granite body, was derived by mixing of a relatively mafic magmatic end-member (87Sr/86Sr∼ 0.7025–0.7050 and δ18O∼ +7.5 to +8.0) with two metasedimentary sources, both having 87Sr/86Sr∼0.715 and δ18O∼ +10.0 to +12.0, but with one being more homogeneous than the other. The more homogeneous component and the (mantle-derived?) magmatic end-member dominate at low structural levels within the complex. The less homogeneous end-member that dominates at high levels is clearly derived from the local Paleozoic pelitic schists. A Rb-Sr age of 330±20 Ma was obtained on hornblende from a deep level within the complex, which fixes this age for the regional metamorphism, as well. Although a post-metamorphic granodiorite magma body at Trois Seigneurs also displays heterogeneities in δ18O and 87Sr/86Sr (and thus does not give a clear-cut Rb-Sr isochron), the data are consistent with an emplacement age between 260 and 310 Ma, similar to ages of other late granodiorites in the Pyrenees. 143Nd/ 144Nd is very uniform within the Hercynian crust, both at Trois Seigneurs (ɛNd=−3 to −7) and elsewhere in the Pyrenees; almost all igneous lithologies have depleted-mantle, mid-Proterozoic model ages, consistent with efficient recycling of crustal material following original crustal accretion in this area at about 1600 Ma or earlier. Rb-Sr mineral ages exhibit a complex cooling history reflecting late Hercynian and Mesozoic thermal events. Our results show that profound homogenization of the 87Sr/86Sr and 18O/16O ratios of large volumes of the crust can occur during regional metamorphism and crustal anatexis, particularly in regions undergoing extensional tectonics. Such processes can significantly modify the isotopic compositions of the protoliths of granitic magmas; this may explain why many peraluminous Hercynian granitoids of Western Europe have anomalously low (87Sr/86Sr) initial values compared to their probable sedimentary parent rocks.

A rifted tectonic setting for hercynian high-thermal gradient metamorphism in the pyrenees

Abstract Hercynian regional metamorphic terrains in the Pyrenees contain evidence of very high-temperature gradients within the crust during metamorphism, with temperatures as high as 700°C attained at 10–12 km below the surface. Stable isotope studies demonstrate that the crust was simultaneously flushed by marine fluids to at least this depth. The absence of any evidence for crustal collision, and the Upper Palaeozoic stratigraphic record for the area, suggest that the tectonic setting for the metamorphism was a zone of continental rifting associated with strike-slip movement. In this zone anatexis occurred at two distinct levels: Cambro-Ordovician pelites at the base of the Palaeozoic sedimentary pile melted to produce per-aluminous magmas, while in the lower Hercynian crust, very large-scale melting generated voluminous granodioritic magmas which then invaded high-structural levels. The thermal structure of the Hercynian crust was profoundly influenced by both convective and advective heat transfer, due to movement of surface derived aqueous fluids, and intrusion of magmas.