Temporal Chemical Variations within Lowermost Jurassic Tholeiitic Magmas of the Central Atlantic Magmatic Province

V.J.M. Salters 1, P.C. Ragland 2, W.E. Hames 3, C. Ruppel 4 , and K. Milla 5

1. National High Magnetic Field Laboratory and Dept of Geological Sciences, Florida State University, Tallahassee, Florida, USA;
2. Department of Geological Sciences, Florida State University, Tallahassee, Florida, USA; Professor Emeritus, present address: 52 Marshall Road, Troy, VA 22974;
3 . Department of Geology and Geography, Auburn University, Auburn, Alabama, USA;
4.  School of Earth and Atinospheric Sciences, Georgia Tech, Atlanta, Georgia, USA;
5. Dept of Environmental Science, Florida A&M University, Tallahassee, Florida, USA.

Abstract

The Central Atlantic Magmatic Province (CAMP), which is of greater extent than any other large igneous province (LIP) yet identified, defined critical stages in the early breakup of Pangea.  The CAMP surrounds the Central Atlantic in eastern North America, northeastern South America, western Africa, and southwestem Europe.  It apparently covers over 7 x 109 km2 and was active for no more than 4 m.y.  Virtually all CAMP rocks are mafic tholeiites, and include both intrusives and extrusives.  The most extensive intrusives are diabase (dolerite) dikes, which occur in three main swarms on Pangaea: NW-, NE-, and NS trending.  These mafic tholeiites, including all intrusives and extrusives, can be classified into three magma types based on their Ti contents: low-Ti (LTi), interinediate-Ti (ITi), and high-Ti (HTi).  The NE swarm contains primarily the ITi magma type, whereas the NW swarm is relatively heterogeneous and contains all three types.  The N-S swarm contains highly evolved (high-Fe) quartz tholeiites in North America and ITi rocks in South America.  Based on both compositions and attitudes, these dike swarms can be correlated across the Atlantic.  For example, the HTi type of the NW swarm is most common in Liberia, and across the Atlantic in adjacent (on Pangaea) Surinam and French Guyana.

The available chemical data do not indicate a simple petrogenetic history for CAMP, and arguments can be made for both "active" and "passive" models of magma evolution.  These complications may in part be due to a lack of sufficiently high-quality geochemical and geochronological data in some critical areas, but they also may be due to a genuine greater degree of complexity for CAMP's evolutionary history compared to other, smaller LIPS.  For example, the relation between known ages and estimates of both depth and degree of melting is contradictory in comparison to other LIPS.  Some patterns within the CAMP are apparent, however.  The most interesting, and potentially significant, pattern is a temporal symmetry of dike and magma characteristics; i.e., characteristics were apparently similar in the early and late stages of the evolution of the CAMP, but quite different in the middle stage.  This symmetry almost certainly has a bearing on the tectonic history of the Pangaean breakup, but at this time its signficance is not clear.  For example, the change of some characteristics with time support an active plume model; the change in others support a passive model.

The two principal magma types within the CAMP, LTi and ITi, were apparently derived from lherzolitic mantle sources that were compositionally similar and contained both continental lithospheric and asthenospheric components.  Compared with other large igneous provinces, the CAMP basalts show depleted geochemical characteristics.  Compositional differences between them are primarily due to differences in depth and degree of melting; LTi represents the deepest and greatest degrees of melting.  We will show that the estimates of degree and depth of melting based on major- and trace-element characteristics can provide important constraints on the mechanism for continental break-up associated with the CAMP.  The temporal progression of the chemical characteristics indicate deeper melting with time, which is consistent with a shallow (such as crustal thinning) and passive origin for the break-up of the Pangaean continent.