TRACE ELEMENTS IN HORNBLENDE REVEAL VARIATIONS IN ENCLAVE-HOST MAGMA INTERACTION IN THE TUOLUMNE INTRUSIVE COMPLEX, SIERRA NEVADA, CA

Microgranular magmatic enclaves (mme) are ubiquitous in all but the most evolved parts of the Tuolumne Intrusive Complex. The mme are typically interpreted to result from magma mixing/mingling on the basis of reversely-zoned plagioclase and exchange of coarse crystals (‘phenocrysts’) from the host into the mme. However, particularly for isolated enclaves, the location(s) of and degrees of hybridization with the host magma are commonly unclear. Here, textures, major- and trace-element compositions, and zoning in hornblende (Hbl) are used to better understand mme origins and evolution.

The mme vary from equigranular to porphyritic, with a wide range of color index. Some are dioritic, lacking groundmass quartz and K-feldspar despite their presence in the host; others are tonalitic to granodioritic. Acicular apatite is very common, but not ubiquitous.

Major element contents of Hbl from mme and host are similar but in many cases trace element abundances are distinct. However, no uniform relationship between trace elements in mme versus host Hbl exists: in some mme–host pairs, Hbl in the mme is slightly more primitive (higher Ni, Cr, V, REE) but the opposite case occurs.

Three examples from the equigranular Half Dome unit illustrate general mme–host relationships. In one, Hbl phenocrysts in mme have REE patterns identical to Hbl in the host, but Hbl clusters (relict host phenocrysts?) are similar to rims of host Hbl. In contrast, groundmass mme Hbl is distinct from the phenocrysts in the mme and the host Hbl (= insertion of host Hbl into mme). In a second, all mme Hbl (phenocrysts and groundmass) share compositions and zoning patterns, whereas host Hbl has distinct REE patterns and lower REE abundances, with the exception of one core which is identical to Hbl in the mme (= scant mme Hbl in host). A third example is a double enclave in which equigranular Hbl in the center of the mme is more primitive than Hbl in the rind or in the host (= partial exchange in rind).

In conclusion: (1) not all mme have the same assemblage as their host, which suggests that diffusional exchange between mme & host melts is not universal; (2) Hbl in mme is not identical to Hbl in the host, and in some cases cannot be related to the host; (3) although in situ exchange of crystals occurs, it is not ubiquitous, suggesting that many mme were entrained from depth.