The Pleasant Bay Layered Gabbro—Diorite, Coastal Maine: Ponding and Crystallization of Basaltic Injections into a Silicic Magma Chamber

• Published in: Journal of petrology 1993-06, Vol.34 (3), p.461-489
• Main Author: WIEBE, R.T.A
• Format: Article
• Language: English
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Igneous and metamorphic rocks petrology, volcanic processes, magmas
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/34.3.461

The Pleasant Bay layered gabbro–diorite intrusion, located on the coast of Maine between Bar Harbor and Machias, is roughly oval in plan, measuring 12 km by 20 km. Gravity data, contact relations, and internal layering suggest that it is basinform in structure with a maximum thickness of ˜3 km. Its roof and upper parts have been lost through erosion. Where it is in contact with underlying granite, the base of the intrusion typically consists of strongly chilled gabbro with convex-downward lobate forms, suggesting that the granite was incompletely solidified when the gabbro was emplaced. Roughly 90% of the exposed rocks are weakly layered gabbro and mafic diorite, both of which vary widely in grain-size and texture. Layers and lenses of medium-grained leucocratic diorite to granodiorite are widely intercalated with the chilled mafic rocks and commonly contain partly digested mafic inclusions; they also commonly contain zones of pillow-like bodies of gabbro chilled on all margins. The dioritic rocks are consistently topped by gabbroic layers with chilled lobate bases and commonly appear to feed granitic pipes and diapirs into overlying gabbro. Much of the intrusion can be subdivided into hundreds of macrorhythmic units (from 1 to 100 m thick) consisting of basally chilled gabbro that grades upward to diorite or highly evolved leucocratic silicic cumulates. Basaltic dikes are abundant both in the underlying granite and in the layered gabbro–diorites; they have appropriate compositions to be feeders for chilled gabbroic layers in the Pleasant Bay intrusion. The layered rocks of the Pleasant Bay intrusion record hundreds of basaltic injections into a chamber with resident silicic magma. Small injections produced chilled gabbroic layers and pillows within silicic cumulates. Larger infusions of basaltic magma produced temporary compositional stratification and episodes of double-diffusive convection within the chamber. Although fractional crystallization produced compositional variation in much of the gabbro, units that grade from chilled gabbro at the base to highly silicic cumulates at the top provide cumulate records of magma stratification and hybridization along a double-diffusive interface between basaltic and silicic magmas. The intrusion provides a superb plutonic record of events that have often been inferred for silicic eruptive centers. Mafic–silicic layered intrusions comparable with the Pleasant Bay are more widespread than has generally been appreci