Tuolumne Intrusive Suite, California: Implications for Magma Rheology
P.J. Jugo, and M.R. Frank
Department of Geology
University of Maryland
College Park, MD 20742
This is a preliminary progress report of a field study, started in the summer of 1995, examining the relationship among textures in high level granites. We mapped two dike complexes in detail in the equigranular Half Dome (HDE) and megacrystic Cathedral Peak (CP) granodiorites of the Tuolumne Intrusive Suite (Map 1). Maps were prepared for the Cathedral Peak on Lembert Dome (LD; Upper and Lower Map Regions), and for the HDE next to Tenaya Lake (TL: Tenaya Map Region). Dikes from both regions exhibit internal textural variability; the dikes of Lembert Dome differ in texture and intrusion behavior from those of Tenaya Lake. Many of the dikes are aplites, pegmatites, or related textural variants, and are mineralogically simple, containing >95% quartz + K-feldspar + plagioclase, with minor biotite, magnetite, titanite, zircon and apatite, and in rare cases, tourmaline, garnet, and secondary epidote and chlorite. Field relations suggest that the dikes at Lembert Dome are locally derived, with diking occurring within a partly molten crystallization interval. We suggest that changes in dikes along strike are a function of changes in host crystallinity and temperature during diking. The locus of dike magma generation at Tenaya Lake is unclear, and the source of melt appears distal rather than proximal.
Oriented grids of 80' by 80' to 180' by 180', were laid out on near horizontal (<18o slope) outcrop surfaces. Dikes were mapped by tape measure/tape and compass, and descriptions were recorded of host rock texture and mineralogy, dike texture and mineralogy, and nature of the contacts.
Lembert Dome Sites
Dikes at these sites commonly exhibit highly variable textures along their length. In some cases, individual dikes can be followed along strike (for 10's of meters) from areas where the dike is wider, has faint or undulose borders, and has textures similar to the groundmass of the host Cathedral Peak, to areas farther north along strike where the dike is thinner, contacts are sharp (more brittle in behavior during fracturing), and the dike texture does not resemble the host texture (Photo 7 and Photo 11). Where dike boundaries are diffuse, host rocks are sometimes marked by concentrations of megacrysts or mafic minerals; these may be sites of dike melt extraction. Where observable, dips are steep and to the east. Many dikes show a distinct asymmetry with a mafic selvage and aplite against the footwall side (Photo 11), and pegmatite concentrated towards the hanging wall side; however, exceptions can be found (Photo 6).
Tenaya Lake Site
Overall, dikes at Tenaya Lake are much more homogeneous along their length than those at Lembert Dome. Texture of HDE dikes is not similar to host rock texture, and dikes are mapped easily by their low color index compared to the HDE host. Contacts are nearly always sharp, and there is evidence for more brittle behavior during diking (e.g. straight borders, right angle bends, and "T" junctions).
In cases where dikes of different textures intersect (here at Tenaya Lake and at a related HDE site at Olmsted Point), the age relationships of the dikes suggest that the transition from normal granitic macrosaccharoidal pegmatitic aplitic texture occurs with increasing temperature difference (undercooling) between dike magma and host rock. The difference in intrusive behavior of dikes between the map areas suggests a difference in host rock (or host rock magma) rheology (e.g. crystallinity) between the Lembert Dome and Tenaya Lake host rocks. Overall, the temperature difference between the dike magma and host magma appears to be closer at Lembert Dome than at Tenaya Lake.
Overview of Dikes in the TIS
Piccoli (1992) estimated that approximately 80% of the dikes along Tioga Road (Yosemite National Park), within the Tuolumne Intrusive Suite, are located within the Half Dome Equigranular and Half Dome Porphyry. This suggests that dikes in each of these units comprise approximately 40 % of the total aplite/pegmatite volume of the Tuolumne Intrusive Suite. The smallest proportion of aplite and pegmatite was found in the Cathedral Peak (0.02%), the largest unit in the Tuolumne Intrusive Suite.
Pegmatitic material is present in three forms: 1) small pegmatites associated with aplitic dikes located throughout the Tuolumne Intrusive Suite; 2) rare pegmatitic segregations, generally less than 0.5 m in diameter, located dominantly within both the porphyritic and equigranular Half Dome; and 3) large pegmatitic segregations (up to 30 m in width) located primarily along the contact of the Kuna Crest with surrounding wall rocks in the May Lake region, and on Johnson Mountain along the contact of the Johnson Porphyry with the Cathedral Peak. The relative proportions of pegmatite to aplite over the entire Tuolumne Intrusive Suite is small.
Apatite in Aplites
Information regarding devolatilization history may be obtained when comparing the halogen content of aplitic apatites and the apatites crystallized from the host rock (the presumed source of the aplite melt). In the Tuolumne Intrusive Suite, Cl/OH is consistently lower in aplitic apatite than host rock apatite. This is not true universally. We found no change in Cl/OH ratios in shallower systems in the Ritter Range. Assuming the aplites were intruded at temperatures lower than or equal to the rocks in which they intruded, it would be expected that Cl/OH would be elevated in the aplitic apatites if the variation in apatite composition was due primarily to a temperature effect. Calculated equilibrium constants indicate that the Cl/OH in apatite increases with decreasing temperature (assuming all other factors remaining constant) indicating that the trend of lower Cl/OH in aplitic apatite relative to host rock might best be explained by an alternative hypothesis, i.e. exsolution of a magmatic volatile phase upon or soon after intrusion of the aplites (Piccoli, 1992).
Estimation of Aplite Intrusion Temperatures
Estimating the temperature at which dikes are extracted or intruded is difficult. In an attempt to overcome these problems, we are exploring the use of apatite saturation temperatures (ASTs) as a proxy for aplite intrusion temperatures. If we make the assumptions that the aplite composition is representative of the melt from which it was derived, and there is no xenocrystic apatite present in these aplites, the temperature at which apatite begins to crystallize can be calculated (Piccoli and Candela, 1994). This second assumption is supported by petrographic analysis; there is no evidence of resorption of apatite, zoning with respect to halogens or major elements, etc., in the samples selected for this analysis,--features which may suggest that the apatite crystallized in some other environment. Several aplite dikes were analyzed for P2O5 and SiO2 so that model ASTs could be calculated. Calculated ASTs for the CP and HDE aplites range from 705-770oC. If these aplites represent residual melt from the apatite-saturated country rock magma, then these ASTs may represent the temperature of extraction of the aplite melt.
Miarolitic texture- Miarolitic cavities exhibit external nucleation of crystals, are spherical or some topological equivalent, and many crystals terminate and project into a void, or a mass of hydrothermally precipitated minerals (in which case the structure is called a miarole). We think that the bubbles of volatile phase that form during isobaric crystallization of a water-saturated melt are small compared to the overwhelming majority of bubbles in medium-grained granites; that's why most granites are not miarolitic. Miarolitic cavities are found in fine-grained rocks (where bubbles grew by decompression to sizes greater that the average grain size), or cases where crystals grow heterogeneously, as in a water-saturated pegmatite (where bubbles grow by coalescence - coalescence also occurs when the volatile proportion is high, as in a foam). Miarolitic cavities may interconnect to form permeability clusters, or elongate, fractal passages of high permeability. Miarolitic structures are the best evidence that a magmatic volatile phase has been exsolved from a magma.
Pegmatite- Externally nucleated; generally, but not exclusively, coarse-grained. This texture can be generated in the presence, or absence, of a magmatic volatile phase. There may be a gradation between pegmatitic and miarolitic texture.
Saccharoidal texture- Internally nucleated allotriomorphic texture (xenomorphic granular). We recognize two variations of this texture. When the rock is fine-grained, we refer to the texture as aplitic. When the rock is medium to coarse-grained, we refer to the texture as macrosaccharoidal. Macrosaccharoidal texture is most easily distinguished from granitic texture (hypidiomorphic granular) by the presence of quartz eyes with or without quartz beading (see Photo 8).
Mafic selvage- Higher concentration of ferromagnesian minerals (relative to the host rock), that occurs along the border of a felsic dike. When within host rock or a dike, this is referred to as schlieren. We have noticed a strong spatial relationship between mafic schlieren and the occurrence of pegmatitic pods. These may be nucleation-driven phenomena (Naney and Swanson, 1980).
Results of the preliminary mapping component of this project suggest:
at Lembert Dome, field relations suggest that dikes were intruded into a partially molten crystallization interval, and dike melts were locally derived;
dikes at Lembert Dome often exhibit extreme textural variability along strike, suggesting changes in intensive parameters (most likely temperature) of the host rock (or magma) that changed along dike length;
at Lembert Dome, diffuse dike boundaries are recognized by sharp changes in the concentration of megacrysts or mafic minerals, and are probable sites of dike melt extraction;
at Lembert Dome, dikes are asymmetric with a mafic selvage and aplite against the footwall side, and pegmatite concentrated towards the hanging wall side; however, exceptions can be found; and,
our working model, based on cross cutting dikes of different textures at Lembert Dome, Tenaya Lake and other areas within the Tuolumne Intrusive Suite, is that the age relationship of the dikes suggests that the transition from normal granitic macrosaccharoidal pegmatitic aplitic texture, occurs with increasing temperature difference between dike magma and host rock.
Photo 1, Photo 2, Photo 3, Photo 4, Photo5, Photo 6, Photo 7, Photo 8, Photo 9, Photo 10, Photo 11
Naney, M.T. and Swanson, S.E. (1980) The effect of Fe and Mg on crystallization in granitic systems. American Mineralogist, v. 65, p. 639-653.
Piccoli, P.M. (1992) Apatite chemistry in felsic magmatic systems. Ph.D. dissertation, University of Maryland at College Park, 295 p.
Piccoli, P.M. and Candela, P.A. (1994) Apatite in felsic rocks: a model for the estimation of initial halogen contents in the Bishop Tuff (Long Valley) and Tuolumne Intrusive Suite (Sierra Nevada Batholith) Magmas. American Journal of Science, v. 294, p. 92-135.