BI-MODAL (TRONDHJEMITIC/GRANITIC) FUSION OF SEDIMENTARY ROCKS ALONG MARGINS OF EARLY JURASSIC DIABASE INTRUSIONS, NEW JERSEY AND NEW YORK

The intrusion of Jurassic diabase into the Triassic sediments of the Newark Basin has resulted in extensive thermal metamorphism and local fusion. The argillitic sediments of the Lockatong Formation have been particularly affected. The most potassic beds have been metamorphosed into black hornfels consisting largely of albite and biotite while the most sodic beds have been metamorphosed into a light gray hornfels consisting largely of albite and diopside. A wide variety of additional hornfels were also generated. Contrasts in the degrees of crystallization of fusion products and diabase have resulted in only minor magma mixing. Some of the fusion products were injected into cooling joints within the diabase as dikes, others were retained within migmatitic zones along sediment-diabase contacts, or retained along the margins of partially fused xenoliths. Partial melting of dark gray potassic hornfels layers (4.0% Na₂O, 6.3% K₂O) has resulted in the development of migmatitic intergrowths of granite or syenite (4.2% Na₂O, 4.6% K₂O) together with a refractory residue enriched in biotite and chlorite (4.6% Na₂O, 6.5% K₂O) at Brookville and Fort Lee, New Jersey. The composition of the granites and syenites plot near the low pressure Quartz-Feldspar minimum of the Quartz-Albite-Orthoclase phase diagram. Fusion of the sodic hornfels was more complex. One large xenolith of sodic hornfels in diabase at Graniteville, New York underwent partial fusion generating a melt depleted in aluminum (only 6.75 % Al₂O₃) but enriched in CaO (13 %). The refractory residue was enriched in Na₂O (7 %) and SiO₂ (75 %) and consists largely of albite and quartz. Subsequent high degrees of partial fusion of the albite and quartz rock has resulted in a trondhjemite melt that has intruded Jurassic diabase bodies along late cooling joints at Secaucus and Fort Lee, New Jersey. The composition of the trondhjemite plots near the eutectic point in the Quartz-Albite system.