WHOLE-ROCK GEOCHEMICAL ANALYSIS OF THE STRAITS SCHIST, WESTERN CONNECTICUT

The whole-rock geochemistry of high-grade Silurian-Devonian metasediments of the Connecticut Valley Zone of western CT and MA have been analyzed using XRF spectrometry. Analyzed rocks include The Straits Schist (DSt), its basal member (Stb), and its upper Southington Mountain member (DSts) (Table 1). Immobile Th and Ti, and high field strength elements (U, Y, Nb, and Zr) were used to evaluate protoliths. The bottom of the sequence is composed of shallow water carbonates finely interbedded with sandstone. The Straits is chemically similar to pelitic and psammitic shales and was deposited as a sandy shale fining upward into an Al-rich shale. The upper part of the sequence, DSts, is a graphite-rich schist with high U levels, characteristic of deep-water black shales. Based on field relations and chemical discrimination diagrams, this sequence of rocks is conformable and is interpreted as arc-derived sediments deposited in the distal extension of a subsiding basin.

ppm	Mo	Nb	Zr	Y	Sr	U	Rb	Cr	V	Ti (wt. %)	Ba	Th
DSts	3.428	19.285	253.027	40.105	28.921	4.471	155.991	95.897	131.362	1.122	618.409	18.296
DSt	2.996	14.960	184.252	34.525	83.907	2.100	147.728	97.605	134.807	0.963	663.524	15.063
Basal	0.853	10.325	130.249	20.846	88.653	0.012	92.083	73.400	71.882	0.514	482.282	7.864
Weight %	Fe	Mn	Ti	Si	Al	Ca	K	P	Mg	Na	LOI	Weight %
DSts	7.405	0.103	1.078	70.641	14.437	0.085	2.808	0.106	1.871	1.496	0.005	100.035
DSt	7.036	0.157	0.992	66.135	17.070	1.122	3.258	0.174	2.253	1.860	0.006	100.063
Basal	5.970	0.116	1.092	60.023	16.594	5.679	2.565	0.149	4.458	3.343	0.002	99.991

Table 1: Average whole-rock geochemistry of the Southington Mountain member (DSts; n=3,2), The Straits Schist (DSt; n=17,16), and its basal member (Stb; n=4,2).