Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...
Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...
Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...
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<strong>activity</strong> was initiated. The initial border faulting and early stages<br />
of basin subsidence apparently did not coincide with any eastern<br />
North American tholeiitic <strong>activity</strong>.<br />
The gross aspects of petrology and geochemistry of the Early<br />
Jurassic eastern North American igneous province are relatively<br />
uniform when compared to the highly varied compositional<br />
range of most volcanic suites associated with island-arc or com-<br />
pressional settings. Despite the large size of the province, only<br />
four major and one minor geochemical populations (Weigand<br />
and Ragland, 1970; Ragland and Whittington, 1983b) account<br />
for the great majority of the igneous occurrences. The uniform<br />
geochemistry of the province implies that the prevailing tectonic<br />
processes were acting uniformly throughout the entire province<br />
or at least extensive parts of it. The geochemical populations<br />
include two olivine-normative types (high LIL and low LIL), two<br />
quartz normative types (high Ti and low Ti), and a minor alkali-<br />
olivine type (Ragland and Whittington, 1983b). Weigand and<br />
Ragland (1970) also recognized an additional quartz-normative<br />
type (the high-Fe type) that Ragland and Whittington (1983b)<br />
. <strong>Post</strong>-<strong>Paleozoic</strong> <strong>activity</strong> 353<br />
now interpret as a variety of the high-Ti type. The normative<br />
groups are included in Table 1.<br />
Olivine Normative Types<br />
The major-element chemistry of the olivine normative tho-<br />
leiites is similar in many respects to mid-ocean ridge basalt<br />
(MORB). The olivine normative tholeiites, however, unlike<br />
"normal" MORB, display a slight chondrite-normalized enrich-<br />
ment of light rare earth elements (REE) relative to heavy REE<br />
(Ragland and others, 197 1; Bryan and others, 1977: Philpotts and<br />
Schnetzler, 1968; Gottfried and others, 1977), suggesting a man-<br />
tle source that was less depleted than the MORB source.<br />
The two olivine normative types are distinguished from each<br />
other on the basis of differences in large-ion lithophile (LIL) trace<br />
elements and iron content (Ragland and Whittington, 1983a).<br />
The texture of the low-LIL olivine diabase is typically sub-<br />
ophitic with very few phenocrysts. Typical low-LIL diabase is<br />
composed of approximately 55 percent plagioclase (Anyo), 30<br />
TABLE 1. EARLY JURASSIC EASTERN NORTH AMERICA IGNEOUS ROCKS (continued)<br />
HIGH-FE QUARTZ NORMATIVE<br />
- TI, Zr, and Cu-depleted basalt and diabase - -TI, Zr, and Cu-enriched basalt and diabase -<br />
HighFe Sander Preakness Holyoke Pomperaug Pal-int. Ladentown Cushetunk Hook Mt. Hampden<br />
No. of<br />
Analyses 15 14 11 10 19 7 5 2 6 6<br />
so2<br />
lo2<br />
4 9 3<br />
Fe2°<br />
Fed<br />
MnO<br />
w<br />
CaO<br />
NaoO<br />
KpO<br />
P2°<br />
H20+<br />
H20-<br />
Total 100.67<br />
Trace elements (ppm)<br />
Ba .......<br />
Co 52<br />
Cr 94<br />
cu 74<br />
Ni 34<br />
Rb 22<br />
Sr 178<br />
v .......<br />
Zn 99<br />
Zr 94<br />
Reference 1