摘要
Deep xenolith are important samples for us to know composition and nature of the deep crust. There are many new volcanos which can all kinds of xenoliths such as a lower crustal graulite, and upper mantle derived peridolite and lherzolite. In eastern China, there occured many deep xenoliths in Mesozoic Cenzoic alkaline basalt. Besides these, some small volcanos eruptions were founded in southern Hunan province, which age is about 132-151 Ma. Because there are many kinds of xenoliths in basalt, many geologists paid attention to them. We studied host basalt and its xenoliths in detail, and obtained some new results as follows: Huziyan basalt belongs to the typical alkaline basalt. While the magma erupted out, it took many xenoliths from deep crust and upper mantle. These xenoliths include: mafic granulite, gabbro and feldspar gneiss as well as mantle drived xenoliths such as pyroxenite, peridotite and spinel lherzolite. The age of granulite and gabbro xenoliths is not definite, for example, gabbro age is 1 141±67 Ma, and the age also may be regarded as 224±24 Ma. The author gained new results by using zircon U Pb dating, and the interceptions of the cordia and discordia are 787±30 Ma and 298±27 Ma. The two ages display two geological events respectively, the former is the age of gabbroic magma formation definetly, the later may indicates the metamorphic age of lower crust. Feature parameters of lithophile element of host basalt and xenoliths are different, i.e., host basalt K *=2K N/(Nb N+La N)=2.08>1, which shows the origin area of magma had suffered the metasomatism, its Nb *=2K N/(Nb N+La N)=0.30<1, which shows the magma hybridized with crustal rocks after it erupted; while the lower crust xenoliths (gabbro and granulite) are different, K * and Nb * of gabbro are 0.94 and 0.54. They just displayed basic magma suffered hybridization after it intruded to crust. K * and Nb * of granulite xenolith are 6.4 and 0.25, which also suggested the strong interaction between upper mantle and lower crust. Its mineral inner isochron age is 544±14 Ma, and its ε Nd =-6.6, these results imply that granulite xenolith has been contaminated by crust. The two stage mode age of Nd reequilibrium is 1 766 Ma, so rock forming age is inferred to be the Proterozoic era. From above discussion, it can be concluded that there are several period of magma intrusive events in lower crust, may be regraded as the middle Proterozoic (1 766 Ma), late Proterozoic (787±30 Ma), metamorphism of granulite and metamorphic event of gabbro may suggest there would be magmatic process between mantle and crust in Caledonian (544 Ma±) and early Indosinian (298 Ma±) period. So the author suggests the intrusion of basic magma and its uderplating to lower crust resulted in the overgrowth of continent crust in southern China. It is a kind of vertical accretion which is quite different from lateral overgrowth apparently.
Deep xenolith are important samples for us to know composition and nature of the deep crust. There are many new volcanos which can all kinds of xenoliths such as a lower crustal graulite, and upper mantle derived peridolite and lherzolite. In eastern China, there occured many deep xenoliths in Mesozoic Cenzoic alkaline basalt. Besides these, some small volcanos eruptions were founded in southern Hunan province, which age is about 132-151 Ma. Because there are many kinds of xenoliths in basalt, many geologists paid attention to them. We studied host basalt and its xenoliths in detail, and obtained some new results as follows: Huziyan basalt belongs to the typical alkaline basalt. While the magma erupted out, it took many xenoliths from deep crust and upper mantle. These xenoliths include: mafic granulite, gabbro and feldspar gneiss as well as mantle drived xenoliths such as pyroxenite, peridotite and spinel lherzolite. The age of granulite and gabbro xenoliths is not definite, for example, gabbro age is 1 141±67 Ma, and the age also may be regarded as 224±24 Ma. The author gained new results by using zircon U Pb dating, and the interceptions of the cordia and discordia are 787±30 Ma and 298±27 Ma. The two ages display two geological events respectively, the former is the age of gabbroic magma formation definetly, the later may indicates the metamorphic age of lower crust. Feature parameters of lithophile element of host basalt and xenoliths are different, i.e., host basalt K *=2K N/(Nb N+La N)=2.08>1, which shows the origin area of magma had suffered the metasomatism, its Nb *=2K N/(Nb N+La N)=0.30<1, which shows the magma hybridized with crustal rocks after it erupted; while the lower crust xenoliths (gabbro and granulite) are different, K * and Nb * of gabbro are 0.94 and 0.54. They just displayed basic magma suffered hybridization after it intruded to crust. K * and Nb * of granulite xenolith are 6.4 and 0.25, which also suggested the strong interaction between upper mantle and lower crust. Its mineral inner isochron age is 544±14 Ma, and its ε Nd =-6.6, these results imply that granulite xenolith has been contaminated by crust. The two stage mode age of Nd reequilibrium is 1 766 Ma, so rock forming age is inferred to be the Proterozoic era. From above discussion, it can be concluded that there are several period of magma intrusive events in lower crust, may be regraded as the middle Proterozoic (1 766 Ma), late Proterozoic (787±30 Ma), metamorphism of granulite and metamorphic event of gabbro may suggest there would be magmatic process between mantle and crust in Caledonian (544 Ma±) and early Indosinian (298 Ma±) period. So the author suggests the intrusion of basic magma and its uderplating to lower crust resulted in the overgrowth of continent crust in southern China. It is a kind of vertical accretion which is quite different from lateral overgrowth apparently.
