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

Field, 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. -- The TLS is a composite intrusion and contains four main zones (Zones I-IV). Layering features (physical and chemical) of the suite reflect the complex interaction of several magma chamber processes including magma replenishment with or without subsequent mixing of newly introduced and residual magmas, in situ crystallization, current deposition and postcumulus/subsolidus modifications. Effects and consequences of these processes vary from place to place within the suite. 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 eNd variation within the suite from -0.1 to +5.1 (at 420 Ma) and much of the range of this heterogeneity appears to have existed before emplacement of magma batches into the chamber which suggests crustal contamination prior to intrusion. -- The FS and W-SCS are geochemically and isotopically distinct, temporally unrelated, high-level granitoid intrusions. The FS has been dated at 420 ±2 Ma and is a C-type, metaluminous granitoid body. It typically displays: i) LREE enrichment relative to HREE with flat HREEN patterns; ii) negative Eu- and Nb- anomalies; and iv) relatively positive eNd values (-1.0 to +1.3). The FS is interpreted to have been mainly derived from lower crustal material. -- The W-SCS has been dated at 408 ±2 Ma and is an H-type, metaluminous, composite intrusion. It contains three distinct units: the Pigeon Island Unit-hornblendite; the Sandy Unit-leucocratic gabbronorite; and the Wild Unit - quartz diorite, tonalite and minor granodiorite. The suite is characterized by i) LREE enrichment relative to HREE with steeply sloping REEN patterns; ii) no notable Eu- anomalies; iii) significant negative Nb- anomalies; and iv) a wide range of Nd isotopic compositions (+0.3 to +3.2). -- The Wild Unit dominates the W-SCS and typically contains various types of mafic-intermediate dykes, microgranular enclaves and country-rock xenoliths. Magma mixing and mingling processes between mantle-derived mafic and lower crust-derived felsic magmas played important roles in the evolution of the Wild Unit. -- 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) → heat transfer ± minor mass transfer (crustal anatexis with or without a minor contribution from mantle-derived mafic magma- the FS)→ to both mass and heat transfer (magma mixing and mingling- the W-SCS).