Population-Level Analysis of Small-Volume Continental Basalt Provinces Using Large Geochemical Datasets

Session: Transforming Earth and Planetary Science Through Data and Data Management: In Honor of MSA Distinguished Public Service Medal Awardee, Kerstin Lehnert

Presenting Author:

Authors:

Abstract:

Large igneous rock geochemical datasets delivered through EarthChem allow volcanic provinces to be investigated at the population level, using statistical tools applied in population analysis, but is an approach not widely applied.  Post‑subduction, small‑volume continental basalt provinces provide a demonstration of the utility of a population-level approach.  Two Cenozoic provinces with abundant whole‑rock compositional data are those in southwestern North America (SWNA) and Tibet.  In both regions, post‑subduction volcanism includes multiple monogenetic alkali basalt centers, each short‑lived and producing <0.1 km³ of eruptive material.  Province‑wide, however, mafic volcanism has been active diachronously for ≥30 Myr and over areas >1 million km².  Volcanism was preceded by long-lived subduction involving ocean-continent convergence in SWNA but culminated in Tibet with the collision of the Indian and Asian continents at ~45 Ma.  In both regions, potassic alkali basalts (including the most primitive rocks, wt. % MgO>6) are recognized as a separate class of volcanic rocks that have low high field strength element (HFSE) to large ion lithophile element (LILE) ratios and high Pb contents characteristic of arc-related magmatism potentially derived from sources in “subduction-modified” continental lithospheric mantle (CLM).  The potassic basalts can be treated as separate volcanic rock populations and large available data sets allow province-wide (aggregate) trace element concentration means and standard deviations to be determined.  These parameters provide information on the style of metasomatism and the composition of the metasomatizing agents.  Aggregate means for incompatible elements (n>~150 per element) in SWNA are 3-7x those of a sediment-free, Pacific island arc basalt (IAB) composite created as a comparator from precompiled datasets from GEOROC but are 25-200x in Tibet.  In both locations, standard deviations correlate with aggregate means, as expected for a set of isolated batch melts of a variably metasomatized CLM source, but the high aggregate concentrations cannot be due solely to lower degrees of melting compared to IAB and require metasomatic additions to CLM.  Enrichments in melt mobile elements in SWNA likely occurred in Precambrian but aqueous fluid mobile element abundances were augmented during low angle subduction in Late Cretaceous.  High enrichments in all melt mobile elements in southern Tibet basalts reflect metasomatism by silicic melts derived from subducted Precambrian crustal material, with relative aggregate abundances requiring melts of compositionally evolved (granitic) crust.