Structure and petrology of the Partridge Island block and its role in the tectonic evolution of the Saint John area, New Brunswick

The Partridge Island block (PIB) consists of highly deformed metasedimentary and meta-igneous rocks in and near the city of Saint John in southern New Brunswick. This study uses geological mapping and petrology to subdivide the block into distinct units and structural analysis to delineate the deformational history of the PIB relative to surrounding units. In the Tiner Point area the PIB consists of variably mylonitized and tectonically interlayered metasedimentary rocks, leucotonalite and alkali feldspar granite that are in steeply faulted contact with highly altered basalt of the Taylors Island Formation. The alkali feldspar granite is A-type and consists primarily of aegirine and K-feldspar; tectonic setting discrimination diagrams indicate that it formed in a within-plate extensional setting. One sample yielded a U-Pb (zircon) date of ca. 340 Ma. In contrast, the leucotonalite consists almost entirely of quartz and plagioclase and displays an arc-like chemical signature. In the Red Head area, the PIB consists of mylonitized quartz monzodiorite and leucotonalite, unconformably overlain by tholeiitic basalt and sedimentary rocks of the Taylors Island Formation and thrust over the Carboniferous Balls Lake Formation. The quartz monzodiorite displays an arc-like signature, interpreted to be the result of hybridization of bimodal magmas, the felsic end member of which is the Tiner Point alkali feldspar granite and its albitized equivalent the Red Head leucotonalite. A 40Ar/39Ar muscovite cooling age of 332±3 Ma for the quartz monzodiorite indicates mylonitization followed by exhumation and Taylors Island Formation deposition, thrusting to the southeast, and finally IOCG-type mineralization. This history, comparable to that of the Cobequid Highlands, resulted from dextral transpression along the Cobequid-Chedabucto Fault Zone during terrane accretion.