GEOCHEMISTRY OF A BORON-RICH PERALUMINOUS GRANITE PLUTON - THE CALAMITY PEAK LAYERED GRANITE PEGMATITE COMPLEX, BLACK-HILLS, SOUTH-DAKOTA

The Calamity Peak pluton is a small, layered granite-pegmatite intrusive body associated with the Harney Peak Granite in the Black Hills, South Dakota. The pluton is texturally and chemically heterogeneous; it consists predominantly of tourmaline - muscovite - garnet granite and pegmatite, along with minor muscovite - biotite - garnet granite, and includes approximately 400 m of alternating granite and pegmatite, in layers 0.1-2.0 m thick. Pegmatite layers are enriched in perthite whereas granite layers contain relatively more albite and tourmaline so that Na2O/(Na2O+K2O) ranges from 0.37 to 0.91. Along with Na, granite layers are relatively enriched in B, Mg, Ca, Fe, Zn, Zr, REE, Hf, Th, and U, whereas pegmatite layers are relatively enriched in K, As, Rb, Nb, Cs, Ba, Ta, and Pb, and have higher Eu/Eu*. Relative to "primitive" biotite granites that occur in other portions of the Harney Peak Granite, rocks at Calamity Peak are enriched in B (approximately 5 x - 80 x), Na, P, Mn, As, Sb, Cs, and Ta, but depleted in Mg, K, Ca, Sc, Ti, Cr, Fe, Co, Cu, Sr, Y, Zr, Ba, Hf, Pb, Th, U, and all REE except Lu. Depletion in thorium and LREE is attributed to early crystallization and separation of monazite. Enrichment in Na at Calamity Peak and the development of prominent igneous layering are attributed to phase-equilibrium shifts induced by the high B content of the magmas. The overall chemical changes are consistent with derivation of the least fractionated Calamity Peak compositions through 70% fractional crystallization of Harney Peak biotite granites; this agrees with structural evidence that the Calamity Peak pluton formed after much of the main Harney Peak Granite dome had been emplaced.