Central Atlantic Magmatic Province
From Wikipedia, the free encyclopedia
The Central Atlantic magmatic province (CAMP) was formed during the breakup of Pangaea during the Mesozoic Era. The initial breakup of Pangaea in early Jurassic time provided a legacy of basaltic dikes, sills, and lavas over a vast area around the present central North Atlantic Ocean.
Contents |
[edit] Connected magma flows
Although some connections among these basalts had long been recognized, in 1988 they were linked as constituting a single major flood basalt province(Rampino and Stothers). The basaltic sills of similar age (near 200 Ma, or earliest Jurassic) and composition (intermediate-Ti quartz tholeiite) which occur across the vast Amazon River basin of Brazil were linked to the province in 1999(Marzoli et al.).
[edit] Geographical extent
The province has been described as extending within Pangaea from present-day central Brazil northeastward about 5000 km across western Africa, Iberia, and northwestern France, and from the interior of western Africa westward for 2500 km through eastern and southern North America(McHone).
If not the largest Province by volume, the CAMP certainly encompasses the greatest area known, roughly 11 million km², of any continental large igneous province.
Nearly all CAMP rocks are tholeiitic in composition, with widely separated areas where basalt flows are preserved, as well as large groups of diabase (dolerite) sills or sheets, small lopoliths, and dikes throughout the province.
Dikes occur in very large individual swarms with particular compositions and orientations. CAMP activity is apparently related to the rifting and breakup of Pangaea during the Late Triassic through Early Jurassic periods, and the enormous province size, varieties of basalt, and brief time span of CAMP magmatism invite speculation about mantle processes that could produce such a magmatic event as well as rift a supercontinent(Wilson; McHone).
[edit] Connection with the T-J boundary and the associated mass-extinction event
Since this argument is still debated within the geological community, it seems a good choice to present the two main points-of-view. One hypothesis is based especially on studies on Triassic-Jurassic basins from Morocco where CAMP lava flows are outcropping(e.g., Marzoli et al., 2004), whereas the other is based on data from eastern North American basins and lava flows(Whiteside et al., 2007), respectively.
[edit] Morocco
[edit] Introduction
The thickest lava flow sequences of the African CAMP are situated in Morocco, where we can find more than 300 metres thick basaltic lava piles. The most studied area is Central High Atlas, where the best preserved and most complete basaltic lava piles are exposed. According to geochemical, petrographic and isotopic data four distinct tholeiitic basaltic units were recognized and can be placed throughout the Central High Atlas: Lower, Intermediate, Upper and Recurrent basaltis. The Lower and Intermediate units are constituted by Basaltic Andesites, whereas the Upper and Recurrent units have basaltic composition. From Lower to Recurrent unit, we observe:
- a progressive decrease of eruption rate (the Lower and the Intermediate units represent over 80 % of preserved lava volume);
- a trend going from intersertal to porphyric petrographic texture;
- a progressive depletion of incompatible element contents in the basalts, possibly linked to a progressive depletion of their mantle source.
[edit] Isotopic analyses
Ages were determinated by 40Ar/39Ar analysis on Plagioclase(Knight et al., 2004; Marzoli et al., 2004; Verati et al., 2007). These data show undistinguishable ages (199.5 +/- 0.5 Ma.) from Lower to Upper lava flows, from central to northern Morocco. Therefore the CAMP is an intense and short magmatic event. Basalts of the Recurrent unit are slightly younger (mean age: 197 +/- 1 Ma) and represent a late event. Consistently, the Upper and Recurrent basalts are separated by a sedimentary layer, that locally reaches thickness of circa 80 m.
[edit] Magnetostratigraphy
According to magnetostratigraphic data, the Moroccan CAMP were divided into five groups, differing in p-mag directions (declination and inclination)(Knight et al., 2004). Each group is composed by a smaller number of lava flows (i.e., a lower volume) than the preceding one. These data suggest that the CAMP were created by five short magma pulses and eruption events, each one possibly <400 (?) years long. All lava flow sequences are characterized by normal polarity, except for a brief paleomagnetic reversal yielded by one lava flow and by a localized interlayered limestone in two distinct section of the High Atlas CAMP.
[edit] Palynological analyses
Palynological data from sedimentary layers samples at the base of four lava flow sequences constrain the onset of the CAMP, since there is no evidence of depositional hiatus or tectonic deformation at the bottom of the lava flow piles(Marzoli et al., 2004). The palynological assemblage observed in these basal layers is typical of Late Triassic age, similar to that of the uppermost Triassic sedimentary rocks of eastern North America(Fowell and Traverse, 1995). Samples from interlayered limestone in lava flows provided unreliable palynological data. In fact only one limestone bed from the top to the central High Atlas upper basalts yielded a Late Triassic palynological assemblage. However, the observed sporomorphs in this sample are rare and poorly preserved.
[edit] Conclusions
All of these data indicate that the basaltic lava flows of the Central Atlantic magmatic province in Morocco were erupted at ca. 200 Ma and spanned the T-J boundary. Thus, it is well possible that there is a connection between this magmatic event and the T-J boundary climatic and biotic crisis, that lead to the mass-extinction.
[edit] References
Articles and Research Papers
- Whiteside, J.H., P.E. Olsen, D.V. Kent, S.J. Fowell, M. Et-Touhami, 2007, Synchrony between the Central Atlantic magmatic province and the Triassic-Jurassic mass-extinction event?: Palaeo, v. 244, p. 345-367.
- Marzoli, A., H. Bertrand, K.B. Knight, S. Cirilli, N. Buratti, C. Vérati, S. Nomade, P.R. Renne, N. Youbi, R. Martini, K. Allenbach, R. Neuwerth, C. Rapaille, L. Zaninetti, G. Bellieni, Synchrony of the Central Atlantic magmatic province and the Triassic-Jurassic boundary climatic and biotic crisis: Geology, v. 32-11, p. 973–976.
- Knight, K.B., Nomade, S., Renne, P.R., Marzoli, A., Betrand, H., Youbi, N., 2004. The Central Atlantic magmatic province at the
Triassic–Jurassic boundary: paleomagnetic and 40Ar/30Ar evidence from Morocco for brief, episodic volcanism. Earth and Planetary Science Letters 228, 143–160.
- Hames, W.E., McHone, J.G., Ruppel, C., and Renne, P., eds., 2003, The Central Atlantic Magmatic Province: American Geophysical Union Monograph 136, 267 p.
- McHone J.G., 2000, Non-plume magmatism and rifting during the opening of the Central Atlantic Ocean: Tectonophysics, v. 316, p. 287-296.
- Marzoli, A., P.R. Renne, E.M. Piccirillo, M. Ernesto, G. Bellieni, and A. De Min, 1999, Extensive 200 million-year-old continental flood basalts of the central Atlantic magmatic province: Science, v. 284, p. 616-618.
- Wilson, M., 1997, Thermal evolution of the Central Atlantic passive margins: Continental break-up above a Mesozoic super-plume: J. Geol. Soc. London, v. 154, p. 491-495.
- Fowell, S.J., Traverse, A., 1995. Palynology and age of the upper Blomidon Formation, Fundy Basin, Nova Scotia. Review of Palaeobotany and Palynology 86 (3–4), 211–233.
- Rampino, M.R., and Stothers, R.B., 1988, Flood basalt volcanism during the past 250 million years: Science, v. 241, p. 663-668.
