There are different opinions on the genesis of lamprophyre. The popular viewpoints are as follows: (1) The upper mantle rock is partially melted under the condition of volatile components such as CO2, which is similar to the cause of kimberlite. 2 From the residual magma forming the granitic rock, the basic magma is separated, and the lamprophyre is crystallized. 3 It is formed by crystallization of volcanic basalt magma. Volatile fractions H2O and CO2 promote the formation, movement, floating and rounding of self-formed phenocrysts such as biotite and amphibole in lamprophyre. 4 The basalt veins are transformed into lamprophyre due to the alkali metasomatism of the hydrothermal gas flow. 5 Magma liquid immiscible or assimilated and mixed, can also form lamprophyre.
Potassium-magnesium lamprophyre is a kind of igneous rock with a porphyry shape, a subvolcano or a spurt. It is chemically rich in K2O and MgO, and sometimes rich in TiO2, but SiO2 is substantially saturated. Its characteristic minerals are leucite phlogopite, potassium-alkali magnesia, and zirconium silicate. The main rocks are diopside white garnet, Bailuo Jinyun porphyry, Jinyun white porphyry porphyry and magnesium iron leucovorin volcanic rocks, which sometimes produce tuff-like appearance. Potassium-magnesium lamprophyre can contain diamonds, and the Argyle Diatreme in western Australia is famous for its potassium-magnesium lamprophyre rich in diamonds (1.03 grams per ton of rock). Since potassium-magnesium lamprophyre is often symbiotic with kimberlite, its origin is related to the formation of kimberlite. It is considered to be a distinct product of kimberlite magma at medium and low pressures.
Through detailed geological, petrological, mineralogical, isotope geochronology, elemental and isotope geochemical studies, and rock geological geochemistry such as the entire strip, the lamprophyre in the Baimazhai nickel ore district is summarized. The genetic information of the genesis and its relationship with the regional potassium-rich igneous rocks; preliminary investigation of the nature of the porphyrite-enriched mantle fluids in the Mazhai nickel deposit area and the age of the occurrence of the enrichment events; quantitative inversion of the partial melting degree of the rocks, The residual mineral phase, source REE content and crystallization differentiation process in the source area were established. The geodynamics model of the lamprophyre in this area was preliminarily established.
The emplacement age of the lamprophyre in the Baimazhai nickel deposit is 32.01±0.60～32.46±0.62Ma, which is the product of the high-potassic magmatic activity in the early Cenozoic fault zone.
2. The lamprophyre in the Baimazhai nickel ore district is an alkaline series, potassium metamorphic-superpotassium calc-alkaline lamprophyre. The fluids deposited from the subduction crust and oceanic crust contribute to the source region of the metasomatic enrichment. The crust mixing effect during the magma evolution is weak, and some melting and crystallization differentiation have an effect on the diagenesis process. According to the REE content, it can be divided into two groups. The elemental geochemical characteristics show that the low REE group lamprophyre has undergone the crystal differentiation of monoclinic pyroxene + olivine + plagioclase ± Fe-Ti oxide ± apatite. The high REE group lamprophyre experienced the crystallization differentiation of olivine + monoclinic pyroxene + plagioclase. The low REE group and the high REE group lamprophyre are respectively products of about 10% and 4% partial melting of the enriched mantle. The mineral proportion of the original magmatic remnant phase of the low REE group of the low REE group simulated by petrological mixing calculation is Ol67.21Opx16.99Cpx11.82Gar4.00. The quantitative simulation calculation of the REE content in the source area indicates that the lamprophyre in the Baimazhai nickel deposit is derived from the rich LREE.
The simulation results of crystallization of the low REE group porphyry show that the low REE group porphyry in the mining area is the product of direct magma crystallization, relatively low crystallization (23.74%) and relatively high crystallization (44.15%). Rock-forming minerals and whole rocks
The geochemical characteristics of the lamprophyre are similar to but different from those of the Machangyu gold deposit, the Beitun gold deposit, the Yao'an gold deposit, the Laowangzhai gold deposit, and the calc-alkaline porphyry, reflecting Ailao. The provenance and magma evolution of the Cenozoic potassium-rich igneous rocks in the fault zone are similar and heterogeneous.
3. Based on regional geology, petrology, mineralogy and geochemistry, the genetic model of the lamprophyre in the Baimazhai nickel ore district was initially established: the Indian plate from about 70 to 50 Ma collided toward the Asian plate, and the subducted fluid (including small The scale melt) accounted for the underground lithospheric mantle of the Yangtze block, forming the enrichment area of ​​the porphyry in the Baimazhai nickel deposit, and proceeded with the subduction, about 40 Ma (the new high-potassic magmatism in the Ailaoshan fault zone begins) At the time), the ancient Tethys plate and the Indian plate that dive into the mantle broke off (Slabbreak-off), causing the asthenosphere to rise. In the background of the conversion tension, the hot sonosphere triggered The partial melting of the lithospheric mantle was previously formed, forming the lamprophyre in the Baimazhai nickel deposit. The extensive potassium-rich igneous magmatism in the region triggered large-scale strike-slip shearing (about 27-22 Ma) in the Ailaoshan fault zone.
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