Petrology and geochemistry of the southern shatter zone, Cadillac Mountain pluton, Mount Desert Island

Coombs, Michelle

'The goal of this study was to look at features of the late silurian Cadillac Mountain Intrusive Complex (CMIC) which had previously been only summarily investigated. The main body of the CMG, an a-type, hypersolvus, hornblende granite, is underlain by a unit of layered gabbro-diorite which is believed to be the result of multiple infusions of mafic magma into the partially crystalline granitic magma of the pluton (Wiebe and Chapman, 1993). Mafic enclaves found through all levels of the CMG are evidence that magma mixing occurred following these infusions.' 'My particular project dealt specifically with the intrusive breccia, or shatter zone, along the southern margin of the pluton. The granitic material which comprises the igneous matrix of the shatter zone exhibits mineralogy and composition which is distinct form typical CMG. The study involved five shatter zone localities along the southern coastline of the island which were studied for field relations, petrography and geochemistry. Particular attention was paid to the transition from shatter zone matrix to typical CMG. Granitic matrix found deep within the shatter zone is rich in biotite, and occasionally in plagioclase, and is free of mafic enclaves. Chilled textures are locally developed within the granite matrix and attest to rapid emplacement within the shatter zone. Strian indicators are locally prominent in quartz. Directly above the shatter zone lies a thin, basal zone rich in plagioclase with subhedral clinipyroxene and fayalite. This cumulate-like layer grades into typical CMG. These mineralogical variations may indicate a sudden change in temperature within the pluton at the time of shatter zone emplacement. Also found within the shatter zone are leucocratic fine-grained injections (FGI) which appear to be highly evolved magmatic liquids emplaced before complete crystallization of the granitic matrix. REE patterns indicate that FGI and MGM originated as components of one magma. Comparisons of CMG compositions with those of shatter zone matrix indicate that magma mixing had limited affect on the chemical composition of Cadillac Mountain Granite.' 'Petrographic textures and limited country rock assimilation indicate that the shatter zone was shallowly emplaced. Evidence for rapid crystallization of shatter zone magmas, coupled with an increase in temperature within the pluton during the formation of shatter zone, suggests that the shatter zone formed suddenly, perhaps due to an explosive eruption triggered by an influx of basaltic magma into the CMB chamber. A model of side-wall crystallization is proposed to explain cumulate textures within the shatter zone, as well as the presence of FGI, which are interpreted as derivatives of MGM. As crystallization occurred along the margins of the pluton, FGI were displaced to the interior. During brecciation of wall rocks, FGI were injected outwards back into the MGM from which they originated.' 'A preliminary investigation of the tectonic setting of the CMG and related rocks was undertaken using chemical compositions of shatter zone rocks in chemical discrimination diagrams. The shatter zone and CMG appear to have been emplaced in an extensional environment, perhaps related to back-arc rifting within Avalon prior to the Acadian orogeny.' /STUDY LOCATIONS:/ /MOUNT DESERT ISLAND/