Study Of Leucogranite Dykes In The Altai Metamorphic Zones

The Chinese Altai is located in the southwestern margin of the Central Asian orogenic belt. Granite and granitic gneiss developed extensively in the region. Leucogranite dykes also occurred in the metamorphic belts. The chemical composition and the age of the leucogranite dykes are critical to the understanding of the origin of the leucogranite and even the geological evolution. The leucogranite dykes in the high grade metamorphic blets were studied by integrating petrological, geochemical and geochronological analyses. The main findings are as follows:(1) The leucogranite dykes are mainly distributed in the high grade sillimanite zones of the metamorphic belt. The dykes without foliation include garnet and muscovite bearing tonalite, garnet bearing two-mica tonalite, two-mica monzonitic granite, garnet bearing two-mica monzonitic granite, garnet and muscovite bearing monzonitic granite, two-mica syenogranite and garnet bearing two-mica alkali feldspar granite. The dykes with weak foliation include garnet and muscovite bearing tonalite, garnet bearing two-mica tonalite, sillimanite and garnet bearing two-mica tonalite, tourmaline and garnet bearing two-mica monzonitic granite and garnet and muscovite bearing syenogranite.(2) Both petrographic and geochemical characteristics show that the leucogranite dykes in the high-grade metamorphic zones belong to peraluminous S-type granite. The main elements result shows the rocks are rich in silicon and aluminum but poor in iron and magnesium with A/CNK from 1.00 to 2.16. The leucogranites dykes without foliation are rich in Cs, Rb, K, Zr, Sn but depleted in Ba, Nb, Eu, Ti. The element P is enriched in muscovite bearing tonalite but depleted in garnet bearing mica monzonitic granite. The leucogranites dykes with foliation are all rich in Cs, Rb, W, U, K, P, Zr but depleted in element Ba, Nb, Eu, Sr, Ti. The total abundance of rare earth elementsr(REE) is relatively low. The fractionation between light and heavy REE of most samples is weak. Most samples have obvious Eu(δEu=0.09~0.59) negative anomaly while only a few display positive Eu anomalies(δEu=1.64~4.90).(3) The leucogranite dykes formed mainly in Triassic(197±12~254.9±5.6 Ma). The ages can be subdivided into stages :(1) ~254 Ma(2) 230~233 Ma,(3) 216~223 Ma, and(4) 197~204 Ma. In general, the monzonitic granite and syenogranite formed earlier than alkali feldspar granite and tonalite.