|
Proterozoic Volcanic Suites of the Guiana Shield |
Department of Earth Sciences, Downing Street, Cambridge University, CB2 3EQ, UK
The dominantly metavolcanic Issineru Formation (IF) forms the lower 5–7 km of the stratotype for the early Proterozoic Mazaruni greenstone belt, N Guyana. Basalts and gabbros comprise the lower part of the section. The upper half includes some basalts, but is predominantly intermediate and felsic rocks. Most of the Issineru Formation volcanics form a tholeiitic series but minor calcalkaline hornblende andesites and two-pyroxene basaltic andesites with alkaline affinities also occur. Issineru Formation tholeiitic basalts have flat, and slightly LREE-depleted patterns with overall abundances of about 10 x chondrites. Some porphyritic basalts have LREE-enriched patterns and may be calcalkaline. The basalts have similar high field strength element (HFS) patterns on multielement MORB-normalized diagrams, with fairly flat patterns and depletion in P and Ti. Their large ion lithophile element (LIL) contents are more variable, due in part to the LIL’s susceptibility to alteration, but range from slightly less than MORB to about 8 x MORB. These patterns are transitional between MORB and island arc tholeiites. Tholeiitic basaltic andesites and andesites have higher REE levels than the basalts and slightly LREE-enriched patterns (Ce/ Yb = 1.3–2). Calcalkaline hornblende basaltic-andesites and andesites have REE patterns with steeper slopes (Ce/Yb=4.2). All of the Issineru felsic rocks have high REE levels (La = 60) and relatively flat patterns with pronounced negative Eu anomalies. Elsewhere in N Guyana there are rhyolites with steeply-sloping REE patterns, which are considered part of the calcalkaline series. The two-pyroxene basaltic andesites with alkaline affinity have very steep REE patterns (Ce/Yb= 10). They may be sills associated with post Trans-Amazonian continental volcanics, rather than coeval members of the greenstone belt volcanism. The IF tholeiitic series can be qualitatively explained by fractional crystallization from a mafic parent with a flat REE pattern.
