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 leucogranites suggests coexistence of two magmas with different temperatures and viscosities. The microgabbros are injected by granite veinlets separating basic rocks into pillow-like fragments. The substance of these veinlets is intensely contaminated, but the outer rims of basic-rock globules exhibit poor hybridization of granites. Microgabbro bodies are composed of oligoclase, hornblende, biotite, K-Na-feldspar, and, less often, late pyroxene. All of these minerals are uncommon to basic rocks. Early magmatic pyroxene and plagioclase occur as relics. 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°C in leucogranite quartz, 710±10°C in quartz of near-contact leucogranites, and 730±10°C in interstitial quartz of hybrid quartz monzonite. For amphibole from hybrid quartz monzonite T_hom of MI is 755±15°C, and for the late pyroxene in microgabbros it is 810-850°C. Low crystallization temperatures and specific mineral composition of the microgabbros indicate that they crystallized under the interaction with the host acid melt. It is shown that, during hybridization, fluid components, K, Rb, LREE, Zr, and some other elements were supplied into basic magma. Most likely, owing to the injection of the acid melt along contraction cracks deep into basic-rock bodies, the contacting surface of two magmas increased, and their interaction became more active. The reason for weak hybridization of leucogranites near the outer rims of basic-rock globules was, apparently, the differences in the rates at which basic and acid substances ascended along the feeder. The acid melt moved through the heavier portions of basic substance, thus removing hybridization products. The substance of granite injections could not renovate during the ascent, which led to strong hybridization of granites in the veinlets, up to formation of quartz monzonites.