Rare earth elements systematics of West Shasta metavolcanic rocks; petrogenesis and hydrothermal alteration

Abstract

West Shasta district bimodal volcanics (Copley Greenstone and Balaklala Rhyolite) are members of a low K calc-alkaline suite. They have rare earth element distributions characterized by pronounced light rare earth element depletion and unfractionated heavy rare earth elements. The striking similarities of the rare earth element patterns between the two compositions suggest derivation by partial melting of the same, previously depleted, source region in the Devonian upper mantle or lower crust. These sources are even more depleted in incompatible elements than the sources for similar low K suites in the southwest Pacific and Caribbean.The basic volcanics appear to be similar to low Ti, normal or I-type midocean ridge basalts. Clinopyroxene (Wo 42 -En 51 -Fs 7 ) now partially altered to chlorite (Mg/Mg + Fe -- 0.73- 0.95) was the apparent high pressure liquidus phase in the basalts. The absence of an Eu anomaly indicates that the source had no intrinsic anomaly and that no significant low-pressure fractionation of plagioclase took place prior to or during the emplacement of the andesitic lavas. The Balaklala extrusions have a pronounced negative Eu anomaly whereas the intrusive, coeval, Mule Mountain trondhjemite does not. Consequently, the anomaly in the Balaklala could not be inherited from the source region but must instead reflect the fractionation of plagioclase in shallow magma chambers prior to eruption.Hydrothermal fluids associated with the formation of volcanogenic massive sulfide deposits have modified the rare earth patterns of the acid extrusions in the immediate vicinity of apparent feeder systems indicated by the presence of stockworks. Europium has been selectively mobilized (probably as a consequence of reduction to the divalent state) and deposited around the vents. The light rare earth elements have, as a group, been selectively extracted by the hydrothermal fluids resulting in their severe depletion relative to the heavy rare earth elements.Oxygen isotope signatures are dominated by regional, low- to moderate-temperature exchanges with seawater. Copley Greenstone delta 18 O values reflect the opposing effects of chloritization (decreasing delta 18 O) and albitization (increasing delta 18 O). The Balaklala Rhyolite tends to be enriched in 18 O relative to the Copley Greenstone due to a higher content of quartz.Hydrogen isotopes show the regional effects of chloritization (light hydrogen) except in the immediate area of hydrothermal vents where a pronounced increase in delta D is observed. The increase in delta D is a consequence of the formation of coarse muscovite in the altered vent rocks.