Petro-stratigraphic analysis of plagioclase-rich lavas in northern Kenya reveal prolonged high-temperature storage of magma in a flood basalt province.

Abstract:

Continental flood basalts (CFB) are among the largest magmatic events on Earth, characteristically manifesting as stacks of monotonous mafic lavas that initially appear devoid of evidence for the evolution of the magmatic system. Closer examination reveals that the lava piles formed through a dynamic pattern of eruptive pulses and hiatuses that may be recorded in the crystal cargo of these lavas. The crystal cargo in CFBs is interpreted as evidence for the presence of transcrustal magmatic systems that undergo episodic recharge and evacuation at multiple crustal levels. However, the residence of magmas at any particular crustal level is unknown. To probe the residence time for magmas in part of a CFB’s plumbing system, we present an examination of stratigraphically constrained plagioclase-bearing lavas associated with the Eocene CFB province in East Africa. The stratiform mafic lavas exposed in northern Kenya consist of alternating aphyric and plagioclase-rich lava packages consistent with periods of eruption punctuated by volcanic hiatus, where magmas stall and crystallize plagioclase at medium to shallow crustal levels. Plagioclase compositions (n=280) exhibit little intra-crystal or intra-sample compositional diversity and are interpreted to be the result of equilibration during prolonged storage at high temperature. Such an interpretation is supported by bulk-traverse equilibrium calculations for Ba and Sr that indicate intra-crystal chemical equilibrium. While Sr may equilibrate in 10s to 100s of years, for slower diffusing elements like Ti and La, equilibration could take 10³ – 10⁵ of years for the observed crystal sizes. We conclude that the residence of magmas in the middle to shallow crust in the Eocene CFB province to be >10⁵ years; an observation consistent with geochronological studies of flood basalt stratigraphy of the Deccan traps in India. This study demonstrates the utility of such methods as an alternate geochronologic probe in CFB provinces.