Compositional Evolution of Tourmaline-Group and Associated Minerals From Pegmatites in the Larsemann Hills, East Antarctica
The Larsemann Hills, Prydz Bay are underlain by medium-pressure, granulite-facies rocks cut by several generations of anatectic pegmatites of Early Cambrian age. Pegmatites cutting metasedimentary rocks containing up to 10,000 ppm B are host to 8 borosilicates: boralsilite (Al16B6Si2O37), werdingite ((Mg,Fe)2Al12 (Al,Fe)2Si4B2 (B,Al)2O37), grandidierite ((Mg,Fe)Al3BSiO9), prismatine ((Vac,Fe,Mg)(Mg,Al,Fe)5Al4(Si,Al,B)5O21(OH,F)), dumortierite ((Al,Vac,Ti)Al6(BO3) Si3O13 (O,OH)2), and three tourmaline-group minerals. Microtextural and chemical study of these minerals in the context of their initial growth and subsequent deformation is key to understanding the evolution of the pegmatites. Using both electron microprobe analyses (B content was assumed to be ideal) and optical petrography, we have identified two generations of dumortierite and three of tourmaline in 8 pegmatite bodies belonging to two generations. Tourmaline is found with three distinct textures: (1) large crystals with primary oscillatory zoning, (2) a graphic intergrowth with primary quartz, and a (3) distinctly secondary phase replacing boralsilite. Tourmaline (1) ranges from schorl-dravite, (Na0.54K0.01Ca0.06Vac0.39) (Fe1.98Mg0.22Al0.66Ti0.03Vac0.10)Al6 (Si5.82Al0.18)O18(BO3)3(OH3.86F0.09Cl0.05), to foitite, (Na0.37Ca0.03Vac0.59)(Fe2.16Mg0.03Al0.72Vac0.08) Al6(Si5.99Al0.01)O18(BO3)3(OH3.9F0.05Cl0.05). Tourmaline (2) is a schorl-dravite averaging (Na0.57K0.02Ca0.33Vac0.07)(Mg1.32Fe1.26Al0.12Ti0.15)(Al5.79Mg0.21)(Si5.80Al0.20)O18(BO3)3(OH3.74F0. 26), whereas tourmaline (3) is foitite averaging (Na0.37K0.01Ca0.05Vac0.57)(Fe2.22Mg0.01Al0.62Vac0.14)Al6(Si5.99Al0.01)O18(BO3)3(OH3.91F0.05Cl0.05). Tourmaline-group minerals systematically show a trend of increasing Al content with increasing proximity to K- feldspar. In addition, the composition of dumortierite varies with texture and distance from K-feldspar, i.e., dumortierite contains less TiO2 (0.00 - 0.84 wt%) in close proximity to K-feldspar than isolated from it (2.45 - 5.45 wt%). Primary dumortierite forms prisms containing little As + Sb + Nb (0.13 wt% average as oxide), whereas secondary dumortierite, commonly found in fractures, is more massive or fibrous, and contains more (0.48 wt% As + Sb + Nb as oxide on average). Prismatine averaging (Vac0.70Fe0.27Na0.02)(Al5.50Mg2.53Fe0.97Mn0.01)(Si3.79B0.88Al0.34Ti0.02)O21(OH0.67F0.33) is found in a graphic intergrowth with tourmaline and quartz, often with biotite rims. Boralsilite in splays, bundles, or individual prisms surrounded by quartz is fractured and partially replaced by tourmaline, whereas boralsilite prisms found in K-feldspar is mostly replaced by unidentified phyllosilicate. Our results suggest that tourmaline and dumortierite grains were fractured and altered by circulating fluids, resulting in multiple episodes of growth of both minerals whose chemical evolution was partially controlled by their proximity to or inclusion in other mineral phases.
EVIDENCE FOR THE EXISTENCE OF A Pb-DOMINANT MEMBER OF THE TOURMALINE GROUP
In our on-going investigation of pocket assemblages in the Minh Tien granitic pegmatite, in the Luc Yen district of Vietnam, we have identified zoned crystals of tourmaline with anomalously high amounts of lead. The crystals are tiny, from 20 to 150 μm in length, and characterized by a prominent compositional zonation. We document a distinct negative correlation between Fe and Pb. Such a chemical variation is also manifested optically, in plane-polarized light, by a bluish iron-rich core with up to 10 wt.% FeO and a colorless, Pb-enriched periphery; the Pb content varies from a few weight percent to as much as 17.5 wt.% PbO (electron-microprobe data). In some crystals, the distribution of Pb is distinctly patchy and irregular, and there are abundant micro-inclusions of quartz. The crystals are confined to fracture-filling veinlets of albite + quartz in crystals of amazonitic K-feldspar, which likely contained close to 0.7 wt.% PbO, but now contain a fraction of that amount where they are transected by such veinlets. The paragenesis is clearly a hydrothermal one, and yet almost a "closed system" in the sense that the zonation is progressive and the Pb enrichment, extreme. The fluid responsible is likely to have involved a mixture of H2O, issued from the crystallizing evolved silicate magma, and CO2, emanating from the ruby- bearing marble wallrocks. We identify the source of the Pb as the enclosing K-rich feldspar, which is expected to expel Pb as the primary disordered K-rich feldspar orders and unmixes in its evolution toward an equilibrium state at low temperature. In our future work, we will seek to document the light elements Li, B and H in the tourmaline, and to characterize the outer portions of the crystal by X-ray diffraction. The Minh Tien pegmatite is a contaminated NYF pegmatite, presumably of anatectic origin. It probably formed during a period of regional reheating and renewed growth of ruby in the host rocks in Oligocene time.
Euhedral and Skeletal Tourmaline in the Stone Mountain Granite, Georgia, USA
The Stone Mountain granite is a fine-grained biotite white mica granite that intrudes high grade schists and gneisses in the Piedmont Province of the southern Appalachian Orogen, about 15 km east of Atlanta, Georgia. The granite contains variable proportions of feldspar and quartz, more white mica than biotite, and accessory amounts of tourmaline, epidote, apatite, zircon and garnet. Diffrention indices are generally over 90 and the granite has several percent corundum in the norm. Earlier workers suggested the Stone Mountain pluton was composite in nature with multiple intrusions of granite in sheet-like masses along the foliation of the host gneiss at pressures of 3-5 kilobars. Coarse-grained (3-5 cm long) skeletal crystals of tourmaline with interstitial feldspar and quartz occur in the Stone Mountain granite. Surrounding the skeletal tourmaline is a white halo of mica-free granite. The granite is host to at least three generations of thin (mms to cms) pegmatite-aplite dikes. Early dikes (ED) resemble the host granite and locally developed concentrations of garnet along the margin that resemble "line rock". Later pegmatite (P) and aplite (A) dikes cut ED. Dikes P and A P contain tourmaline along with feldspar, quartz and white mica. Beryl and thulite (pink clinozoisite) are also reported from these dikes, but yet identified in this study. Tourmaline occurs as euhedral crystals in both the P and A dikes. Skeletal tourmaline also occurs on the margins of the P dikes. The skeletal tourmaline is Fe-rich (FeOT about 10.5 wt. %) with a dark brown core and a blue overgrowth (colors in plane polarized light). Magnesium and titanium contents of the brown core (MgO = 4.2 wt. % and TiO2 = 0.9 wt. %) are higher than the blue rim (MgO = 3.6, TiO2 = 0.4 wt. %). The skeletal tourmaline crystals display a morphology typically associated with crystallization from undercooled melts. Halos depleted in Fe that surround the skeletal tourmaline crystals formed as the growing tourmaline depleted the local melt environment in Fe, suggesting a magmatic origin for the skeletal crystals. The transition from skeletal tourmaline crystals in the granite and margins of the pegmatites to euhedral crystals in the pegmatite cores and later aplites, provides a model for crystallization of a magma that was initially undercooled and became less undercooled during its late-stage crystallization.