METASOMATIC TRANSFORMATIONS IN TWO METAMORPHIC COMPLEXES DOMINATED BY HETEROGENEOUS Ca-PYROXENE - HORNBLENDE GRANITE AND SYENITE, OTTER LAKE AREA, QUEBEC, CANADA

Abstract

The Bellmount Complex (28 × 5 km) and the Polish Hills Complex (6 × 3 km), found in a high-grade metamorphic terrane in the western Grenville Province of the Canadian Precambrian Shield, consist mainly of granitic–syenitic rocks with highly variable microstructure and mineral proportions. In a 40-meter section, the mean mineral content is Qtz 21 Kfs 55 Pl 14 Cpx 10 Ttn . The atom ratio Mg/(Mg + Fe 2+ ) in Ca-pyroxene and hornblende is equal to 0.6 to 0.7, Na/(K + Na) in discrete crystals of K-feldspar is equal to 0.06, and Na/(Na + Ca) in plagioclase is 0.93. Complex exsolution-induced microstructures formed during cooling to ca. 400°C. The granite and syenite are interlayered with amphibolite, K-feldspar – biotite (veined) gneiss and other rock components. There are signs of metasomatism on the 1 to 10 meter scale within the two complexes. K-feldspar amphibolite formed from Ca-pyroxene granulite with an addition of K, Rb, Sr, and Ba, and removal of Ca and Na, leaving concentrations of Si, Ti, Al, Fe, Mg, P, Sc, V, Zr, Nb, Cs and Hf unchanged. Also, bodies of Ca-pyroxene skarn and Ca-pyroxene – scapolite skarn, possibly formed from dolomite marble with an addition of all major elements except Mg and Ca, and the addition of several trace elements, including V, Zn, Rb, Y, Ba and rare-earths. Furthermore, a mixture of K-feldspar amphibolite and hornblende granite has possibly formed by an alteration of common amphibolite to K-feldspar amphibolite (with an addition of K and Rb), followed by the local transformation of K-feldspar amphibolite to hornblende granite (with an addition of Si and removal of Fe, Mg, Mn and Ca), leaving concentrations of Al, Na, K, Rb, Sr, Y and Ba unchanged. The introduction of elements at numerous places in the two complexes was possibly facilitated by deep fractures of a northeast-trending rift structure, marked by the presence of several bodies of ca. 1.15 Ga nepheline syenite.