EARLY JURASSIC QUARTZ NORMATIVE MAGMATISM OF THE EASTERN NORTH AMERICAN PROVINCE: EVIDENCE FOR INDEPENDENT MAGMAS AND DISTINCT SOURCES

TOLLO, R.P., Department of Geology, George Washington University, Washington, DC 20052 and GOTTFRIED, D., U.S. Geological Survey, Reston, VA 22092

    Compositions of diabase dikes, basalt lava flows, and the chilled margins of intrusive diabase sheets from Virginia to Massachusetts define a suite of at least four quartz normative magma types associated with the Early Mesozoic rifting of eastern North America. These quartz normative magmas, all of which have been recognized previously, include 1) high titanium (HTQ: Ti02 > 1.0 wt.%), 2) low titanium (LTQ: Ti02 = 0.7-0.8 wt.%), 3) high iron (HFQ: Fe203T 13-15.wt.%), and 4) high iron-titanium (HFTQ: Fe203T >,-15 wt.%, Ti02 > 1.3 wt.%) types. New trace element data summarized below, indicate that the four magma types are distinguished by different combinations of the elemental ratios 100Nb/Ti, Th/Hf, Hf/Ta, and La/Yb (chondrite normalized).
 

Trace Element-Ratios (averages - basalts)
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Magma Type     100Nb/Ti     Th/Hf      Hf/Ta      La/Yb(n)

HTQ                 0.10             0.85         4.72         3.21

LTQ                 0.07             0.95         6.08         1.68

HFQ                 0.07             0.94         6.67         2.07

HFTQ               0.05             0.78         7.98         1.44


    Th/Hf and 100Nb/Ti are not effective discriminants between the LTQ and HFQ types. Detailed crustal extension traverses through closed-system diabase sheets indicate that all of the above ratios remain essentially constant throughout an extended range (3-12 wt.% MgO) of fractionation. As a result, these new trace element data provide strong evidence that the regionally extensive quartz normative magma types are not related by differentiation, as proposed by previous investigators. In addition, models invoking an LTQ to HFQ lineage involving pyroxene fractionation are inconsistent with observed higher Sc values in the HFQ type. Furthermore, proposed HTQ to HFTQ lineages are not supported by the observed lack of significant enrichment in the light rare earth elements (notably) La, Zr, Ta, and Th. The stratigraphy of the basalt sequences, which can be broadly correlated between basins, indicates that the different magma types we erupted cyclically and may have been penecontemporaneous. However, the differences in incompatible trace element ratios negate possible derivation from a single zoned magma chamber. The observed consistency of selected trace element ratios with fractionation suggests that these data may be characteristic of the magmatic source materials. We propose that these regional magma types were derived from heterogeneous source materials and remained largely independent during ascent, shallow-level emplacement, and eventual eruption. The data demonstrate that interpretations based on trace element concentrations must involve a combination of elements with a range of geochemical properties in order to distinguish subtle magmatic variations. In this case, the data further underscore the need or distinguishing the effects of source area heterogeneity from complex melting processes in order to understand the origin of the diverse magmas that characterized the early stages of he Mesozoic rifting of eastern North America.