Graduation Year

2024

Document Type

Open Access Senior Thesis

Degree Name

Bachelor of Arts

Department

Geology

Reader 1

Jade Star Lackey

Reader 2

Nicole Moore

Abstract

Xenoliths entrained in recent (Miocene) volcanics from the Sierra Nevada provide insights into a continuously evolving upper mantle during post-batholith generation periods (early Cenozoic–present). Hypothesized mantle refertilization followed by subsequent delamination of the lithospheric root beneath the Sierra provides a contextual framework for analyzing how the upper mantle responds to such events. This study contributes new geochemical data from the ultramafic xenoliths of Blue Knob, including bulk-rock major and trace element data, thin-section petrography, mineral chemistry and thermobarometry, and oxygen isotopes (delta18O). Blue knob (Eastern Sierra) data are compared to previous works on xenoliths from Chinese Peak and Big Creek (Central Sierra) to address changes in thermodynamics and geochemistry on a geographical basis within the Sierra. The Big Creek (9.9 Ma) xenoliths present the most significant bulk-rock geochemical deviation from the primitive mantle (high CaO, Na2O, and Al2O3) and exhibit cold (<800°C) equilibration with the upper mantle. Big Creek also displays lower percentages of partial melting (~15% due to Cr-numbers <40) and the highest delta18O ratios (6.0–9.8‰) which indicate significant crustal contamination. Chinese Peak (10.2 Ma) hosts xenoliths that also show similar degrees of crustal contamination (delta18O ratios: 5.7–8.4‰) but from the hot (>800°C) upper mantle. Blue Knob (8.6 Ma) samples xenoliths with geochemical profiles most like the primitive mantle (high MgO with increasing Na2O, CaO, and Al2O3) and least crustal contamination (delta18O ratios: 4.8–6.5‰). The Blue Knob xenoliths equilibrated at high temperatures (>800°C) and show two significant groups of high (~35%) and low (~20%) partial melting sets (Cr-numbers: 30–90). This locational shift suggests that crustal contamination from residual fragments of the delaminated lithospheric root is continuously interfering with the upper mantle west of the Panthalassan-North American boundary. Conditions east of the boundary indicate the presence of an uncontaminated upwelling asthenospheric mantle actively replacing unoccupied space left by the delaminated root. This study provides significance for addressing how the upper mantle may be continuously evolving beneath the Sierra despite the halt in voluminous batholith production.

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