Monzonitic plutons, California, and a model for generation of alkali-rich, near silica-saturated magmas
✍ Scribed by Calvin F. Miller
- Book ID
- 104748644
- Publisher
- Springer
- Year
- 1978
- Tongue
- English
- Weight
- 691 KB
- Volume
- 67
- Category
- Article
- ISSN
- 0010-7999
No coin nor oath required. For personal study only.
✦ Synopsis
Petrochemical data suggest that early Mesozoic alkali-rich monzonite plutons in California were generated by partial melting of a garnet-bearing, virtually feldspar-free source which was rich (compared to abyssal basalt or sub-oceanic mantle peridotites) in LIL elements. These monzonites are characterized by high Na20 + K20 (near 10 wt% total) and Sr (1000 to 2000 ppm), moderate SiO2 (53 to 63 wt%), steep negatively sloping REE patterns (from >100 to < 10 • abundance), and moderate initial 878r/86Sr ratios (0.705 to 0.707). Most are very slightly oversaturated in silica, but one pluton includes both oversaturated and undersaturated units.
The simplest petrogenetic model which can account for the compositions of these rocks involves partial melting of quartz eclogite. Any differentiation of the resulting magma must take place under pressures at which plagioclase is unstable. The eclogite could be equivalent to a wide variety of continental or oceanic island basalts.
Rocks which are chemically similar to these monzonites, though not extremely abundant, are widespread in both space and time and occur in a wide variety of tectonic settings. These rocks are anomalous in terms of crustal compositions and phase equilibria, but many could be explained as partial melting products of eclogite. Relatively minor thermal events or early stages of major events may tap alkalic intermediate magmas from eclogite pockets as the lowest temperature melting fractions of the upper mantle. granitic rocks of the batholithic belt of California. These plutons are not comagmatic with the more typical granitic intrusions, but rather represent the tapping of a distinct source (e.g., Miller, 1977 b; Sylvester et al., in press).
Approximately silica-saturated alkalic rocks such as monzonites, syenites, latites, and trachytes have been emplaced in a wide variety of settings throughout much of geologic time. In addition to mineralogy and major element composition, analyses of such rocks commonly reveal similarities in trace element chemistry including high incompatible element concentrations and strongly light element-enriched REE patterns (e.g.,