FRACTIONATION, HYDROTHERMAL ALTERATION, AND WALL-ROCK CONTAMINATION OF AN EARLY JURASSIC DIABASE INTRUSION - LAUREL-HILL, NEW-JERSEY

Laurel Hill, an almost completely exposed early Jurassic volcanic neck , consisting of three concentric zones whose diverse chemical composit ion overlaps each of the major eastern North American (ENA) Jurassic d iabase types. Zone 1 is an irregular, heterogeneous, border-zone whose chemistry resembles the low-Ti-quartz-normative type (LTQ) ENA diabas e but is interpreted as an alteration product of high-Ti-quartz-normat ive type (HTQ) diabase. Dilution of HTQ magma with hydrothermal serici te, and with plagioclase and pyroxene that accumulated during fraction ation, has reduced the TiO2 content of Zone 1 from HTQ (1.1%) to LTQ l evels (<0.9%). Localized assimilation of Passaic Formation siltstone i s recognized by an enrichment of Zr in some Zone 1 samples. The chemis try in Zone 2 diabases, the intermediate zone, is typical of HTQ-type diabase. Zone 3, the interior zone, resembles the HFQ-type (High-Fe-qu artz-normative) ENA diabase and is interpreted as an HTQ fractionation product. Although the existence of distinct ENA diabase types from in dependent magma batches is well documented, these processes may transf orm some other diabase or basalt occurrences from one type into anothe r and may therefore explain some otherwise anomalous space-time associ ations.