T118 Proterozoic Crustal Evolution of Southern Laurentia: Tectonic Settings, Petrology, Geochemistry, and Geochronology (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

CHARACTERIZING THE METASOMATIC AGENT IN PERIDOTITE XENOLITHS FROM KNIPPA, TEXAS, USING TRACE ELEMENT AND ISOTOPIC COMPOSITIONS

Paper number: T118-8
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

Spinel peridotite xenoliths in Late Cretaceous basanites from Knippa, TX are the only samples of Texas continental lithospheric mantle. The xenoliths are typically medium-grained spinel lherzolites and harzburgites with equigranular textures and triple junction grain boundaries. Most are fresh but some have serpentine veins. Mineral assemblages and compositions (olivine Fo89-92, orthopyroxene En88-91, Cr-diopside En45-49Fs3-5Wo40-45, spinel Cr# = 15 - 36) record at least one depletion event caused by melt extraction. Primitive mantle normalized whole-rock data for fresh as well as serpentinized samples are variably enriched in light rare earth elements (LREE; (La/Yb)n = 0.26 – 17.1) with flat heavy rare earth element (HREE) patterns (0.80-0.93 xPM). Whole-rock REE as well as in clinopyroxenes show two patterns: (1) LREE enriched (La/Yb)n = 3.47-25.7; (2) spoon shaped (La/Yb)n = 1.97-27.6 and overall flat HREE distributions (1.03-3.63 xPM).

Modeling whole-rock and clinopyroxene trace element abundances shows that the lherzolites can be produced by 3-9% fractional melting whereas harzburgites require 10-15% fractional melting of primitive mantle and agree with estimates based on spinel compositions. Batch melting is not realistic because this requires harzburgites to be produced by 27-30% melt extraction. This should exhaust CPX but Knippa harzburgites contain < 5% CPX. Clinopyroxene Nd and Hf isotopic data show that the Sm-Nd system was disturbed, whereas Lu-Hf results give an age of 359±90 Ma, indicating that the Lu-Hf isotopic composition was completely reset by the Pennsylvanian Ouachita orogeny (~ 350 Ma) during the final stages of assembly of Pangea. LA-ICP-MS Re-Os analyses (TRD 0.28-1.1 Ga and TMA 0.42- 1.42 Ga) on interstitial sulphides also indicate that Mesoproterozoic mantle was reset by a Late Paleozoic event, which may have been metasomatic. PM-normalized trace element troughs at Nb-Ta, Zr-Hf and Ti suggest carbonate melt metasomatism. However, (La/Yb)n vs Zr/Zr* as well as (La/Yb)n vs Ti/Eu indicate carbonate and silicate metasomatism.