Post-Paleozoic activity - Lamont-Doherty Earth Observatory ...

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