The interaction of magma with continental crust at convergent margins is fundamental to
understanding if and how continents grow. Isotopic and elemental data constrain the
progressive stages of development of the magmatic underpinnings of the long-lived
Aucanquilcha Volcanic Cluster (AVC), situated atop the thick continental crust of the
central Andes in northern Chile. Whole rock data are used in conjunction with mineral
compositions to infer processes that gave rise to eleven million years of intermediate,
dominantly dacite, arc volcanism. A pulse of volcanic activity at the AVC between ~5
and 2 Ma is bracketed by more sluggish rates. We document chemical changes in the
lavas that accompany this eruptive evolution. Trace element data suggest that crystal
fractionation and magma mixing were the dominant mechanisms generating the diversity
observed in the AVC whole rock data. Fractionation was dominant during early and
waning stages of magmatism, and magma mixing was an important process during the
high flux period. Peak thermal maturity of the AVC underpinnings coincided with the
high magma flux and likely promoted open system processes during this time. Mineral
compositions from zircon, amphibole, pyroxene, and Fe-Ti oxides confirm the
importance of material recycling in the production of evolved AVC rocks. Various
geothermometers were employed to calculate the pre-eruptive conditions of AVC magma
using mineral compositions. Pressure estimates from amphibole and two-pyroxene
barometry indicate crystallization depths of 1 ��� 5 kb and 4 ��� 6 kb, respectively.
Temperature estimates from zircon, Fe-Ti oxides, amphiboles, and pyroxenes indicate
temperatures ranging from ~700��C to 1100��C. Zircon temperatures are always the
lowest (700��C - 950��C), and pyroxene temperatures are always the highest (1000��C -
1100��C), with Fe-Ti oxide and amphiboles temperatures falling in between. U-Pb ages
from zircons and thermometry from individual samples evidence the thermal maturation
and consolidation of the underpinnings below the AVC, presumably culminating in a
large, crystal-rich mush zone where magmas were trapped and processed. It is in these
middle to upper crustal zones where magmatic diversity is attenuated and giant, relatively
homogeneous batholiths are formed. Isotopes of AVC lavas are similar to values
observed from other central Andes volcanic centers. Lead isotopes are consistent with
the AVC's location within a Pb isotope transition zone between the Antofalla and
Arequipa basement terranes. Oxygen and Sr isotopic ratios are high and Nd isotopic
ratios low with respect to a depleted mantle. Through time, ������Sr/������Sr values of AVC
lavas progressively increase from lows of ~0.70507 to ~0.70579 (upper values of 0.70526
to 0.70680), and ��Nd values decrease from highs of -1.0 to -4.6 (lows of -1.6 to -7.3).
Similarly, O isotopes (�������O) show a slight increase in base level through time from lows
of 6.5��� to 7.0��� (highs of 6.75��� ��� 7.5���). Dy/Yb and Sm/Yb ratios also increased
systematically from highs of 2.11 to 3.45, and 2.76 to 6.67, respectively. Despite the
temporal isotopic variation, there is little isotopic variation with indices of fractionation,
suggesting this signal is the consequence of deep magmatic processing, here attributed to
an expanding zone of melting, assimilation, storage, and homogenization (MASH) of
mantle-derived magma in the deep crust. Upward expansion brought the MASH zone
into contact with rocks that were increasingly evolved with respect to Sr and Nd isotopes,
explaining the isotopic shifts. Downward expansion of the MASH zone enhanced garnet
stability during basalt fractionation, explaining the increased Dy/Yb and Sm/Yb ratios.
Mass balance calculations involving Sr, Nd, and O isotope modeling are consistent with a
crustal component making up 10 - 30% of AVC lavas, implying that although the history
of central Andean magmatism is replete with large scale crustal recycling, the current
phase is largely a crust formation event. / Graduation date: 2012
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/23292 |
| Date | 20 July 2011 |
| Creators | Walker, Barry Alan |
| Contributors | Grunder, Anita |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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