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.