Petrology of the composite mafic-felsic plutonic rocks of the Fogo Island Batholith: A window to mafic magma chamber processes and the role of mantle in the petrogenesis of the granitoid rocks.

摘要

ield, petrographic, geochemical, geochronologic (U/Pb) and Nd isotopic studies of the diverse plutonic rocks in the Tilting area, Fogo Island, NE Newfoundland, were carried out to elucidate the relative roles of crust and mantle in the petrogenesis of this example of Siluro-Devonian magmatism in the Appalachian-Caledonide orogen.;The plutonic rocks of the study area are part of the Fogo Island Batholith (FIB) and display a wide range of fabrics, textures, mineralogies and compositions. Based primarily on field and petrographic characteristics they have been divided into three main suites, these are: the Tilting Layered Suite (TLS) composed of layered ultramafic, mafic and intermediate rocks; the Fogo Suite (FS) dominated by monzogranite with minor amounts of granodiorite; and the Wild-Sandy Cove Suite (W-SCS) containing mainly quartz diorite and diorite with minor amounts of leucogabbronorite, hornblendite and granodiorite.;Two distinct magma compositions have been recognized in the TLS (quartz-tholeiitic, Zones I-III and olivine-tholeiitic, Zone IV). The interrelationship between silica-saturated and silica-oversaturated magma types is attributed to differences in pressure of partial melting of the same or similar source material(s). Variation in water pressure and silica activity are significant parameters controlling the sequence of crystallization within the suite.;The TLS shows slight to strong LREE enrichment relative to HREE and significant negative Nb-anomalies. There is significant ;The FS and W-SCS are geochemically and isotopically distinct, temporally unrelated, high-level granitoid intrusions. The FS has been dated at 420 ;The W-SCS has been dated at 408 ;The proposed petrogenetic model envisages that the TLS, the FS and the W-SCS are the products of the same mechanism, i.e. mafic magma underplating the lower crust. Each suite represents a significantly different mode of interaction between mafic magmas and the lower crust. The mode of such interactions varies from mass transfer (i.e. crustal contamination- the TLS)