Hybridization in composite basic-rock-leucogranite dikes of the Ust'-Khilok massif (Transbaikalia)

Composite basic-rock-leucogranite dikes of the Ust'-Khilok syenite-granite massif have been studied. The character of contacts of microgabbros with le ucogranites suggests coexistence of two magmas with different temperatures and viscosities. The microgabbros are injected by granite veinlets separati ng basic rocks into pillow-like Fragments. The substance of these veinlets is intensely contaminated, but the outer rims of basic-rock globules exhibi t poor hybridization of granites. Microgabbro bodies are composed of oligoc lase, hornblende, biotite, K-Na-feldspar, and, less often, late pyroxene. A ll of these minerals are uncommon to basic rocks. Early magmatic pyroxene a nd plagioclase occur as relies. Studies of melt inclusions (MI) in minerals of leucogranites, hybrid rocks, and microgabbros showed regular changes in their homogenization temperatures. For example, T-hom of MI were estimated at 670+/-10 degreesC in leucogranite quartz, 710+/-10 degreesC in quartz o f near-contact leucogranites, and 730+/-10 degreesC in interstitial quartz of hybrid quartz monzonite. For amphibole from hybrid quartz monzonite Them Of MI is 755+/-15 degreesC, and for the late pyroxene in microgabbros it i s 810-850 degreesC. Low crystallization temperatures and specific mineral c omposition of the microgabbros indicate that they crystallized under the in teraction with the host acid melt. It is shown that, during hybridization, fluid components, K, Rb, LREE, Zr, and some other elements were supplied in to basic magma. Most likely, owing to the injection of the acid melt along contraction cracks deep into basic-rock bodies, the contacting surface of t wo magmas increased, and their interaction became more active. The reason f or weak hybridization of leucogranites near the outer rims of basic-rock gl obules was, apparently, the differences in the rates at which basic and aci d substances ascended along the feeder. The acid melt moved through the hea vier portions of basic substance, thus removing hybridization products. The substance of granite injections could not renovate during the ascent, whic h led to strong hybridization of granites in the veinlets, up to formation of quartz monzonites.