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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Several studies of the intrusions were made early in the<br />
twentieth century, culminating in a thorough paper by Watson<br />
and Cline (1 9 13), who described alkalic dikes of kaersutite camptonite,<br />
"teschenite" (analcite monchiquite?), nepheline syenite,<br />
and "granitic" felsite, as well as numerous intrusions of olivine<br />
dolerite and a few bodies of quartz gabbro. Johnson and others<br />
(1971) describe mica peridotite and pyroxenite in the province,<br />
and intrusions that may be called kimberlite are found to the<br />
southwest and northeast of the province (Plate 5A).<br />
Although all of the intrusions were originally assumed to be<br />
Triassic or older (Watson and Cline, 1913), more recent petrologic<br />
and radiometric work indicates different ages. Zartman and<br />
others (1970) dated minerals separated from a nepheline syenite<br />
dike near Staunton, Virginia, at 145 and 153 Ma by K-Ar methods<br />
(old decay constants) and 114 Ma by Rb-Sr isotopes, or<br />
roughly Late Jurassic-Early Cretaceous. However, many or most<br />
of the Shenandoah Mountain intrusions are apparently middle to<br />
late Eocene in,age^as shown by Rb/Sr and K/Ar dates for more<br />
than a ddn examples, with ages ranging between 42 Ma and 47<br />
Ma (Wampler and Dooley, 1975; Ressetar and Martin, 1980).<br />
Both mafic and felsic intrusions are represented in the Eocene<br />
data. It therefore seems likely that if the Staunton dike is actually<br />
Jurassic, it is not a member of the group and that the only "true"<br />
Shenandoah Mountain province magmas are Eocene.<br />
TECTONIC SETTING OF MESOZOIC<br />
IGNEOUS EVENTS<br />
The reactivation of <strong>Paleozoic</strong> and older Appalachian struc-<br />
tures is apparent in the alkalic Jurassic and younger igneous<br />
provinces in eastern North America. In particular, a series of<br />
'Â¥:-2 + K20 MGO<br />
Figure 26. AFM ternary diagram showing the average Monteregian<br />
plutonic trend and the average Monteregian camptonite, compared with<br />
the Hawaiian alkaline trend.<br />
' <strong>Post</strong>-<strong>Paleozoic</strong> <strong>activity</strong> 361<br />
high-angle fracture zones, major faults, and geophysical linea-<br />
ments is present both parallel with and perpendicular to the<br />
central and western Appalachians (Plate 5A). In many cases, the<br />
linear features could represent important deep-lithospheric<br />
boundaries, such as basement sutures and ancient, buried rifts<br />
(King and Zietz, 1978). The cross-trending features are parallel to<br />
or colinear with oceanic fracture zones off the east coast of North<br />
America (Plate 5A).<br />
The dike orientations and pluton arrangements in eastern<br />
and southern New England appear to be controlled by northeast-<br />
trending fractures and structures aligned with north-south linea-<br />
ment structures, especially for the Early Jurassic members,<br />
whereas the Early Cretaceous intrusions of northwestern New<br />
England and southern Quebec are aligned with east-west to<br />
northwest joints and lineaments. The northwest-trending linea-<br />
ments apparently were the latest associated with Mesozoic<br />
igneous <strong>activity</strong>.<br />
Consistent with this scenario, the trend of the Cretaceous(?)<br />
New England seamounts extends into New England along a<br />
northwesterly line (Plate 5A) and aligns with Line F of Diment<br />
and others' (1980) geophysical lineaments. Along the same zone,<br />
Wise (1982) notes major northwest-trending joint and strike-slip<br />
fault sets in northwestern Massachusetts. Although lamprophyre<br />
dikes are known in southern New England, they are rare, and no<br />
post-Jurassic ages of intrusions have been determined.<br />
MAGMATIC-TECTONIC MODEL<br />
Rather than relying on undemonstrated deep-mantle<br />
plumes, Mesozoic magmatism in eastern North America can be<br />
explained by lithospheric fracture tectonics that are modeled after<br />
well-established rift-related events so evident in East Africa, the<br />
Rhine Graben, the Rio Grande, and other rift areas of continents.<br />
The argument for fracture controls on magmatism is supported<br />
by igneous and structural patterns and satisfies several not solved<br />
by the fixed "hot-spot" model.<br />
The principal magmatic feature, the huge eastern North<br />
American dolerite/basalt province, is actually part of an even<br />
larger zone that in Early Jurassic time extended from present-day<br />
Spain and Morocco (Manspeizer and others, 1978) to northern<br />
South America, over an area roughly 4,600 by 1,500 km. No<br />
single hot spot can explain either the lack of age progression of<br />
magmatism along this zone or its geometric relation to the Ap-<br />
palachian orogen. The absence of any progressive north-south<br />
change of chemistry (Table 1) is also inconsistent with an early<br />
Jurassic hot spot located at either end of the eastern North Amer-<br />
ican province. The intrusions and flows are virtually confined to<br />
structural basement provinces of the eastern Appalachians and<br />
equivalent areas on the other side of the Atlantic. Many of the<br />
Early Jurassic rifting patterns reflect Appalachian-parallel<br />
structures.<br />
Although roughly contemporaneous with eastern North<br />
American dikes, Early Jurassic intrusions of mildly alkalic plu-<br />
tons in New England (the White Mountain Magma Series) are