Cooling pattern and mineralization history of the Saint Sylvestre and western Marche leucogranite pluton, French Massif Central: I. 40 Ar/ 39 Ar isotopic constraints

Details Cooling pattern and mineralization history of the Saint Sylvestre and western Marche leucogranite pluton, French Massif Central: I. 40 Ar/ 39 Ar isotopic constraints Cooling pattern and mineralization history of the Saint Sylvestre and western Marche leucogranite pluton, French Massif Central: I. 40 Ar/ 39 Ar isotopic constraints

Dec 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.4653s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 60, Issue 23, pp.4653-4671

Computer Science

1

Scientific paper

The Saint Sylvestre-western Marche Leucogranite Complex (LC) is a composite pluton emplaced 324 m.y. ago in the Variscan continental crust of the northwestern Limousin, French Massif Central. Petrographic, structural, and new isotopic data are used to constrain the cooling pattern of the pluton in the temperature range for Ar retention in muscovite (Part I) and the genetic relationships with the subsequent uranium mineralization postdating the time of intrusion by 40-50 m.y.(Part II). A suite of thirty-seven muscovite and nine biotite concentrates selected over the entire plutonic complex were analyzed by the 40 Ar/ 39 Ar technique. Muscovite is characterized by well-defined plateau ages ranging between 314 and 301 Ma, while biotite gives less well-behaved age spectra with apparent ages varying between 320 and 300 Ma. The muscovite age variations are systematic and clearly associated with regional trends at the scale of the pluton. Field, petrographic, gravimetric, and structural relationships indicate that the age pattern is controlled by the intrusion shape and the regional, post-cooling fault network cutting through the laccolith. When restored to their approximate pre-faulting geometry, the age variations are shown to correlate positively with sample elevation, with a top-to-base difference as large as 10-14 m.y. over a mean intrusion thickness of ~3 km. This pattern is interpreted to reflect cooling driven by erosion and exhumation of the thickened Variscan crust during the Westphalian and is confirmed by two-dimensional thermal modelling (Part II). The data show that 40 Ar/ 39 Ar mica ages can postdate the time of crystallization of the host pluton by up to 23 Ma depending on the level sampled at the surface; they warn against interpreting K/Ar mica ages from similar, low-relief igneous settings as closely postdating intrusive events without a precise knowledge of the thermal structure, cooling mechanism, and post-cooling faulting of the pluton. In turn, provided that the pre-faulting geometry of the intrusion is properly assessed, the vertical cooling pattern can reveal fine-scale variations in the thermal structure of old eroded intrusions that may be extremely useful for recovering quantitative information on the mechanisms and rates of cooling in ancient orogens.

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