Origin of unusual biotite-apatite rich enclaves, Achala Batholith, Argentina

DAHLQUIST, J.A.; BELLO, C.; ZARCO, J.; ALASINO, P.H.

Ávila

Simposio; Seventh Hutton Symposium on Granites and Related Rocks; 2011

Universidad de Granada

The Achala batholith is a large granitic body emplaced during the Middle-Late Devonian (379–369 Ma) in the Sierras Pampeanas of Argentine (proto-Andean foreland). In the Cañada del Puerto area, A-type granites of this batholith bear unusual enclaves (Dorais et al. 1997; Rapela et al. 2008) containing 89–93% biotite and 7–11% apatite, yielding distinctive compositions: high FeO/MgO ratios (mean 3.1) and very high F content (mean 5.7%), respectively. Accessory minerals are abundant zircon and scarce oxides. Dorais et al. (1997) stated that ‘to the best of our knowledge, no such enclave type has been previously described from other localities’ and they proposed a fractional crystallization origin. Textural studies confirm an igneous origin for the enclaves. Their modal mineralogy produces bulk-rock compositions that cannot represent melts, with very low SiO2 content (32.83 %), high P2O5 values (5.39 %) and very high concentrations of .REEs (4,492 ppm), U (187 ppm), Zr (2,581 ppm) and Y (617 ppm). Based on petrological and geochemical data two main granitic facies were recognized in the Cañada del Puerto area: porphyritic felsic granodiorites and monzogranites. The enclaves are hosted in the former with sharp contacts, and are elongated, tabular or ellipsoidal bodies ranging from few meters to tens of meters in length with thicknesses of up to 1 meter. Fractional crystallization calculations using major oxides are consistent with a biotite-apatite rich enclave such as sample NPE-10 (32.83 % SiO2) being an early-crystallized or “cumulate” assemblage (5 % crystallization), and a monzogranite such as NPE-12 (73.01 % SiO2) being a differentiated melt (95 % melt proportion), each derived from an assumed parental magma equivalent to the porphyritic felsic granodiorite NPE-6 (71.10% SiO2). However, such as cumulate segregation of biotite and apatite has not been previously reported in the literature. Alternatively, liquid immiscibility could be invoked, considering that there is no thermodynamic difference between the separation of crystals or of globules of secondary melt. In both cases, the parent melt becomes saturated with respect to the new phase and precipitates it (Roedder, 1979). Although the distribution of major elements during immiscibility yields a granitic melt resembling that from crystal fractionation, there are important differences in the partition of minor elements (Roedder, 1979). In particular, Roedder & Weiblen (1971) found that P is strongly partitioned into lunar high-Fe melts by a process of liquid immiscibility, the opposite to that found in terrestrial crystal fractionation, where P is concentrated in the more differentiated granites. This situation is observed in the biotite-apatite enclaves and their host granitic rock, where the enclaves are strongly enriched in P as well as U, Zr or Y and REEs, a typical characteristic of melt yielded by liquid immiscibility (Eby, 1979). The 'Ndt values of the granitic rocks and enclaves are indistinguishable: NPE-10, a typical biotite-apatite enclave, has 'Ndt = -5.8, and the average value observed for six granitic rocks is 'Ndt = -5.8. Coupled with textural and mineralogical characteristics and chemical compositions, this strongly suggests that the parental granodiorite magma split into two immiscible liquids to form the monzogranites and biotite-apatite rich enclaves.2 content (32.83 %), high P2O5 values (5.39 %) and very high concentrations of .REEs (4,492 ppm), U (187 ppm), Zr (2,581 ppm) and Y (617 ppm). Based on petrological and geochemical data two main granitic facies were recognized in the Cañada del Puerto area: porphyritic felsic granodiorites and monzogranites. The enclaves are hosted in the former with sharp contacts, and are elongated, tabular or ellipsoidal bodies ranging from few meters to tens of meters in length with thicknesses of up to 1 meter. Fractional crystallization calculations using major oxides are consistent with a biotite-apatite rich enclave such as sample NPE-10 (32.83 % SiO2) being an early-crystallized or “cumulate” assemblage (5 % crystallization), and a monzogranite such as NPE-12 (73.01 % SiO2) being a differentiated melt (95 % melt proportion), each derived from an assumed parental magma equivalent to the porphyritic felsic granodiorite NPE-6 (71.10% SiO2). However, such as cumulate segregation of biotite and apatite has not been previously reported in the literature. Alternatively, liquid immiscibility could be invoked, considering that there is no thermodynamic difference between the separation of crystals or of globules of secondary melt. In both cases, the parent melt becomes saturated with respect to the new phase and precipitates it (Roedder, 1979). Although the distribution of major elements during immiscibility yields a granitic melt resembling that from crystal fractionation, there are important differences in the partition of minor elements (Roedder, 1979). In particular, Roedder & Weiblen (1971) found that P is strongly partitioned into lunar high-Fe melts by a process of liquid immiscibility, the opposite to that found in terrestrial crystal fractionation, where P is concentrated in the more differentiated granites. This situation is observed in the biotite-apatite enclaves and their host granitic rock, where the enclaves are strongly enriched in P as well as U, Zr or Y and REEs, a typical characteristic of melt yielded by liquid immiscibility (Eby, 1979). The 'Ndt values of the granitic rocks and enclaves are indistinguishable: NPE-10, a typical biotite-apatite enclave, has 'Ndt = -5.8, and the average value observed for six granitic rocks is 'Ndt = -5.8. Coupled with textural and mineralogical characteristics and chemical compositions, this strongly suggests that the parental granodiorite magma split into two immiscible liquids to form the monzogranites and biotite-apatite rich enclaves.