The magmatic architecture of continental flood basalts I : Observations
from the Deccan Traps
Abstract
Flood basalts are some of the largest magmatic events in Earth history,
with intrusion and eruption of millions of km$^3$ of basaltic magma
over a short time period ($\sim$ 1-5 Ma). A typical
continental flood basalt (CFB) is emplaced in hundreds of individual
eruptive episodes lasting decades to centuries with lava flow volumes of
10$^3$- 10$^4$ km$^3$. These large volumes have logically
led to CFB models invoking large magma reservoirs ($>$
10$^5$-10$^6$ km$^3$) within the crust or at Moho depth.
Since there are currently no active CFB provinces, we must rely on
observations of past CFBs with varying degrees of surface exposure to
develop and test models. In the last few decades, significant
improvements in geochronological, geochemical, paleomagnetic,
volcanological, and paleo-proxy measurements have provided
high-resolution constraints on CFB eruptive tempo - the volume,
duration, and frequency of individual eruptive episodes. Using the
well-studied Deccan Traps as an archetype for CFB systems, we compile
multiple lines of evidence - geochronology, eruption tempo, dike spatial
distribution, intrusive-extrusive ratio, geochemical variations, and
volcanological observations - to assess the viability of previous
models. We find that the presence of just a few large crustal magma
reservoirs is inconsistent with these constraints. Although observations
from the Deccan Traps primarily motivate our model, we discuss
constraints from other CFBs to illustrate that this conclusion may be
broadly applicable, with important implications for interpreting CFB
geochemical datasets as well as the timing and volumes of
climate-altering volatile emissions associated with CFBs.