Mingling of mafic and felsic magmas in the southern California batholith (Lake Perris, CA)
copyright CIN-TY LEE
This page contains a mapped transect of mafic enclave density and mafic index of the felsic host. click
You can also access the simple mafic index program in Excel here.
And a mafic index cheat sheet for the field here.
1. Undeformed hornblende-rich mafic clast within coarse-grained tonalitic magma. Biotite-rich reaction rind on exterior portion of the clast. Clast is fine-grained suggesting a quench texture. Angular shape suggests an origin by brittle fragmentation of a larger mafic body. click
2. Subrounded hornblende-rich mafic clasts within coarser-grained tonalite magma. Biotite-rich reaction rind on exterior portion of both clasts. Shear stresses imposed by the more viscous tonalitic magma appears to be peeling away the biotite reaction rind, and is in the process of generating a rip-up clast. Note the different thicknesses of the biotite-rich reaction rinds, which may suggest that the margins of the clasts were exposed to the tonalite at different times. Fuzzy clots of schlieren within the tonalite body may represent partially equilibrated/homogenized rip-up clasts. click
3. A large mafic septa within the tonalite body (California Buckwheat bushes Eriogonum fasciculatum for scale). click Close up of the septa with student for scale click. Note that the septum is actually composed of tightly packed mafic clasts, each separated by a tonalitic interstitial magma. Note also that the majority of the clasts are not angular, but subrounded to well-rounded. Clast sizes are poorly sorted. Clasts also show different mafic indices, indicative of different extents of chemical re-equilibration with host tonalite.
4. Mafic lenses and tabular bodies set within a granitic host magma. Macroscale foliation defined by subparallel orientation of mafic lens as well as subtle orientation of mafic minerals and schlieren within the granitic body itself. Mafic lenses show variable rheologic behavior. At center, mafic lense is showing semi-brittle behavior as evidenced by the listric faults, cutting obliquely to the foliation. Where the listric faults occur, the mafic layer is thinned, resulting in incipient necking and boudinage. Granitic melts infiltrate along these faults, resulting in biotitization of the mafic layer. Deformation is most likely dominated by pure shear, with the largest compressional stress oriented perpendicular to the overall foliation (local simple shear within the brittle faults themselves). More ductile behavior is seen in the lighter gray mafic layer in the upper right as well as the mafic layer on the left. Note that the mafic layers are in general much more fine-grained than the felsic host. click
5. Dioritic transition zone within the pluton, showing highly attenuated mafic bodies, now manifested in the form of schlieren. Post-magmatic fault transtensional fault filled with leucogranite. Right lateral sense of displacement. Note the small rip-up fragment near center of photo. click
6. Highly attenuated and partially equilibrated mafic layers within granitic host. Almost all deformation is ductile. However, darker mafic clasts, with higher mafic index, show remnants of brittle behavior in the form of angular margins (clasts in upper left). These clasts appear to be rheologically strong as they do not show significant evidence for ductile deformation. Magmatic foliation, exmplified by the subparallel thin mafic bands, drape around these larger, stronger and darker mafic clasts. Overall deformation seems to be dominated by pure shear with first principle compression oriented perpendicular to overall foliation. Large clasts show little to no evidence of rotation. click
7. Close up of host tonalite body, consisting of alkali feldspar (pink hues), plagioclase (whitish hues), quartz (clear, white hues; minor), and fine-grained biotite and hornblende. Note relatively equigranular grain size of feldspar grains. Mafic minerals in some places occur as clots. Isotropic fabric. Scale of lower field of view is 6 inches. click
8. Swarm of highly attenuated mafic enclaves, showing dominantly ductile behavior. Dominant deformation mode is pure shear. click
9. Non-homogeneous deformation of mafic enclave in tonalite host. Large mafic lens in center of diagram appears to be rheologically stronger in its core than on its margins. Only the margins appear to be participating in ductile deformation and coupling to the deforming tonalite. Slight asymmetry to the "wings" suggest that the mafic clast has rotated slightly and this suggests some amount of simple shear component to the deformation. click
10. Large, rheologically strong mafic clast undergoing semi-brittle deformation as illustrated by the tonalite-filled oblique fault crosscutting the mafic clast. Mafic bands within the tonalite drape around the large, dark mafic clast. click
11. Large swarm of mafic enclaves. Enclave shapes are pancake-like. In this view, we are likely looking perpendicular to the "pancake", which shows minimal large-scale foliation. However, in cross-sectional view, as seen in left, "pancake" enclaves appear to be subparallel, defining a macroscale foliation. click
12. Large angular clast of hornblendite. Note highly angular margins and lack of reaction rind. Mafic clast is very fine-grained. This is likely an example of a large rip-up clast of quenched basalt. Note post-magmatic dike-filled tensional fault crosscutting both the mafic enclave and the tonalite. click
13. Large-scale view of the quarry, showing subvertical mafic septum. Terrace height ~30 feet. In detail, septum is composed of 80% mafic clasts, rather than being a homogeneous layer of quenched basalt. click
14. Mafic enclaves exposed in natural outcrop, high up in the magma body. Differential weathering causes mafic enclaves to either stick out as knobs or fall out, leaving depressions. These enclaves do not show high degree of attenuation based on their angular and equidimensional shapes. Bushes on ground are Encelia farinosa and are about 2-3 feet high. click
15. Attenuated mafic enclaves, looking in the plane of foliation. Note macroscale foliation, but lack of grain-scale fabric in the granitic host. click
16. Image of the Bernasconi Hills, looking northeast towards the San Bernardino Mountains on the other side of the San Andreas and San Jacinto Faults. Boulders represent the integrated aftermath of deep weathering along fractures and joints in the pluton. click
17. Slightly attenuated mafic enclaves. click