Fractionation, Hydrothermal Alteration, and Wall-Rock Contamination of an Early Jurassic Diabase Intrusion: Laurel Hill, New Jersey

Abstract

Laurel Hill, an almost completely exposed early Jurassic volcanic neck, consisting of three concentric zones whose diverse chemical composition overlaps each of the major eastern North American (ENA) Jurassic diabase types. Zone 1 is an irregular, heterogeneous, border-zone whose chemistry resembles the low-Ti-quartz-normative type (LTQ) ENA diabase but is interpreted as an alteration product of high-Ti-quartz-normative type (HTQ) diabase. Dilution of HTQ magma with hydrothermal sericite, and with plagioclase and pyroxene that accumulated during fractionation, has reduced the content of Zone 1 from HTQ (1.1%) to LTQ levels (<0.9%). Localized assimilation of Passaic Formation siltstone is recognized by an enrichment of Zr in some Zone 1 samples. The chemistry in Zone 2 diabases, the intermediate zone, is typical of HTQ-type diabase. Zone 3, the interior zone, resembles the HFQ-type (High-Fe-quartz-normative) ENA diabase and is interpreted as an HTQ fractionation product. Although the existence of distinct ENA diabase types from independent magma batches is well documented, these processes may transform some other diabase or basalt occurrences from one type into another and may therefore explain some otherwise anomalous space-time associations.