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Australia: The Land Where Time Began |
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LIPs - The Panjal Volcanics and Emeishan flood basalts
The oldest and most deformed of known CFBPs from the Phanerozoic are the
Panjal Volcanics. Covering an area of approximately 12,000 km2
of NW reached a peak of thickness of 2.5 km in the Kashmir Valley
(Honnegger et al., 1982). The
main phase of volcanism is marked by basaltic lavas and lesser volumes
of rhyodacitic tuffs, although minor alkali volcanics persisted in the
Kashmir region into the Early Triassic (Veevers & Tewari, 1995). Lower
and Middle Permian subaerial to marginal marine strata are overlain by
basalt lavas and they are in turn overlain by Late Permian (Kazanian)
pelagic strata (Nakazawa et al.,
1975). It is suggested by this facies change that their emplacement is
probably related to rifting and the development of a distal passive
margin (Papritz & Rey, 1989). Estimation of the original volume of the
province is made problematic by the subsequent deformation of the Panjal
volcanics, and metamorphism to amphibolite grade, though it is not
likely the region constituted a “super province” of a size that is
comparable to that of the Deccan Traps.
The eruption of the Emeishan flood basalts in South China was
contemporaneous and comparable in volcanicity. These are also associated
with a probable rifting event (probably the Qiangtang Terrane from the
South China Block, which began in the Early Permian, and they also
subsequently deformed during the closure of Tethys (Chung et
al., 1998). Their outcrop,
which consists of flood basalts with lesser flows of picritic flows and
trachytic and rhyolitic tuffs, covers 3.3 x 105 km2
of SW China (Chung et al.,
1998; Jin & Shang, 2000). The lavas average 705 m thick and they reach 5
km in thickness locally. It has been estimated (Yin et
al., 1992) that the original
volume of the province was 0.6 x 106 km3, while
half this figure was suggested (Jin & Shang, 2000).
It has yet to be determined what the precise age of Emeishan flood
basalts province is. The lavas rest on carbonates from the Middle
Permian (the Maokou Formation) and
marine strata from the Late Permian and Early Triassic directly overlie
them (Yin et al., 1992);
Chung & Jahn, 1995). The presence of fusulinid foraminifera has been
reported recently (Jin & Shang, 2000), from shallow marine limestones
that are interbedded by basal lava flows, which indicates unequivocally
a Guadalupian (Middle Permian) age. Magnetostratigraphic results that
were obtained from a thin (532 m thick) development of the flood basalt
in western Guizhou Province indicates that the lower 450 m had been
erupted during a single, chron of normal polarity and the upper 82 m
during a reversal (Huang & Opdyke, 1996). Therefore, the Meishan CFBP,
as was the case with the Panjal volcanics, appear to have been erupted
over a brief interval that was suggested (Huang & Opdyke to be less than
1 Myr) around the Middle to Late Permian boundary. Acidic ash bands are,
however, widespread in Permian-Triassic boundary sections in South
China, and they are widely regarded to have been derived from the
Emeishan Province (Yin et al.,
1992; Chung &Jahn, 1995; Chung et
al., 1998), which implies there was a prolonged interval of acidic
volcanic activity which spanned the entre Late Permian and lasted into
the Early Triassic. Thick, coarser-grained acidic-intermediate tuff
bands that are 15-25 m thick are also known from the boundary interval
in south Primorie (SE Siberia), which suggests that this area lay closer
to the eruptive centre than South China, where the tuffs are finer
grained and are thinner by an order of magnitude (Kozur, 1998).
According to Wignall it is clear that more work is needed on the
provenance of these P-Tr boundary tuffs.
Attempts to relate the Emeishan flood basalts in [?to] to end-Permian
mass extinction event (e.g. Yin et
al., 1992) are probably
inappropriate, though it is suggested by the dating from the region that
it may be implicated, Along with the Panjal Volcanics, with the
end-Guadalupian mass extinction event (Hallam & Wignall, 1997). This
earlier event, which has been identified within the last few years, had
a primary effect on equatorial marine invertebrate taxa, notably the
fusulinid foraminifera (Leven, 1993; Jin et
al., 1994; Stanley & Yang,
1994). The best of the data was from the southwestern US and South
China, 2 places where the extinctions appear to be related to major
regional regressions which eliminated the shallow marine carbonate
habitats which were the repository of much marine diversity in the
Middle Permian. It was suggested (Hallam & Wignall, 1999) that the
Middle-Late Permian boundary may mark an all-time low point of the sea
level in the Phanerozoic. This sea level fall within South China itself
could reflect regional doming that occurred prior to the eruption of the
Emeishan flood basalts (Chung et
al., 1998), though on a global scale some other mechanism must be
invoked.
Wignall, P. B. (2001). "Large igneous provinces and mass extinctions."
Earth-Sci. Rev. 53: 1-33.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |