大别－苏鲁造山带与华北陆块东部岩石层三维速度结构

The 3-D velocity images of the crest and upper mantle beneath the region of 112° -124°E, 28°-39°N including the Dabie-Sulu orogenic belt are reconstructed by using 36405 P-wave arrivals of 3437 regional and 670 distant earthquakes during the period from 1981 to 1996, and gridding the area of 0.5° * 0.5°. The results of tomography demonstrate that: 1. The results of tomographic imaging show a broad heterogeneity in P wave velocity structure for the lithosphere beneath the Dabie-Sulu orogenic belt. 2. In the Dabie orogenic belt, the velocity patterns in the crust are different among various tectonic units. The Dabie and Qinling orogenic belts are remarkable in the tomographic images, and in mm the Hongan and Dabie blocks in the Dabie orogenic belt are also imaged very distinguishably. 3. A velocity (about 5.9～6.0 km/s) layer exists in the Dabie block at depth between 15～25 km, which is coincident with the low-resistance layer at the depth of 12-23 km, being inferred to be the tectonic detachment zone and suggesting that the extension detachment structure was formed in the middle crust. Beneath the southern and northerm Dabie tectonic units, the north-dipping high-velocity (at level of 6.5 ～ 6.6 km/s) block was developed in the crust, which might be correlated with the UHP rockswith low content of the meta-ultramafic rocks. This result is in agreement with the geological observation on the surface. 4. The velocity image at 40 km depth reveals the features at the top of mantle and the configuration of the Moho discontinuity. The depth of the Moho changes slightly along the trend of the orogenic belt. It in Hongan block is less than 40 km, but it is different in the western and eastern parts of the Dabie block, the former is more than 40 km, and the latter less than or equal to 40 km. The remnant of the mountain root exists between the Shangcheng-Macheng fault and the line of Huoshan-Yuexi-Yingshan in the Dabie orogenic belt, and beneath the southern and northern Dabie tectonic units. However, the thickness of the Moho is about 40 km and there is no obvious changes, which suggest that the Dabie orogenic belt has been experienced quite in the gravity equilibration. The Moho's depth in the Sulu is less than 40 km. 5. There is a dipping slab-like high-velocity body in the uppermost mantle. It is sandwiched by slow velocities and exists beneath the Dabie-Sulu orogenic belt in the range of depths between the Moho discontinuity and 110 km at least. This high-velocity body outlines a picture of the slab interpreted as the remnant of the Triassic subducted YZ. 6. The Sulu orogenic belt displays "crocodilian" velocity structure, the upper crust of the Yangtze thrusted over the Huabei crest, and the Huabei crust indented into the Yangtze crust, where the ancient subduction zone of the Yangtze lithosphere located. Based on the previous geological data, this structure is not related with the collision between the Yangtze and Sino-Korean Blocks, but caused by the sinistral offset of the Tan-Lu Fault. Studied on the velocity structure of the eastern Huabei lithosphere indicates: 1. The 'present-day' lithosphere of the eastern Huabei is between 40-100 km thick with greatly thinned lithosphere around the Bohai Sea. Generally, thickness of the lithosphere in this region decreased eastwards. 2. The attenuation of the lithosphere is attributed to the strongly uplift of the asthenosphere. In the area between the Taihang Mountains and the Tan-Lu Fault, there is a 'lever' with red low velocity belt, it is clearly defined, transverse continuity, depth between 100-150 km, local variations visible, and an upwards trend towards the Bohai Sea. Generally, the velocity structure in the mantle beneath the lithosphere displays irregular column-shape consisting of alternating high and low velocities, and when cold high velocity ancient lithosphere connects with the hot low velocity mantle materials forming precipitous compact structure. More heat pathways from the mantle occur towards the Tan-Lu Fault. 3. The strongly irregular characteristics of the contact between the asthenosphere and the lithosphere is induced by the long-term hot, chemical erosion and alteration on the contact. 4. There are still preserved high velocity lithosphedc root beneath Huabei with 'block-shape' distribution and surrounded by hot materials. Results of our studies indicate that the evolution models of the eastern China mantle are characterized by the direct contact between the uplifted lithosphere and the Huabei Craton accompanying the upwelling of the deep mantle materials. At the contact betwen the lithosphere and the asthenosphere, the upwelled mantle materials replaced and altered the lower lithosphere forming the metasome through the hot and chemical modifications impacted on the Craton lithosphere, and changed it into the lithosphere gradually, resulting in the lithospheric thinning. Thus, the lithospheric thinning is the result of the upwelling of the asthenosphere.

山东莱芜中生代高镁侵入岩及其超镁铁质岩捕掳体的成因研究

In order to know better about the Phanerozoic lithosphere thinning process of Sino-Korea Plate, four Cretaceous intrusion complexes and their ultramafic xenoliths are investigated by this thesis, which are located in Laiwu, Shandong Province, Eastern China, a region far away from plate margin. The four complexes, Kuanshan, Jiaoyu, Jingniushan and Tietonggou, intruded into Archaeozoic granite gneiss and Paleozoic carbonate rocks with scam iron ore at their contact zone. The four complexes can be divided into two magma series, abyssal rocks for the early and hypabyssal rocks for the later. K-Ar dating show that the abyssal rocks intrusion began with 120 ±2 Ma and the hypabyssal rocks intruded about 113 Ma. Abyssal rocks, mainly made up of augite diorites, amphibole diorites and gabbros for the lesser, are chemically characterized with high-Mg (Mg#>0.5) high-K calcalklic rock, which are depleted with Nb, Ta and Ti related to LILE and extremely enriched with Sr and Pb. Comparatively, augite diorites are the most LREE enriched in abyssal rocks, and they show no Eu abnorrnity or weak positive Eu abnormity. Gabbros show the least LREE enrichment with a strong Eu abnormity relatively. In (~(87)Sr/~(86)Sr)_1 -ε Nd(T) diagram, the abyssal rocks show a mixing trend , a rapid change in ε Nd(T) with a relatively small change in (~(87)Sr/~(86)Sr)_1. Low radiogenic Sr and Pb composition with high radiogeic Nd composition indicate that the mixing processes happened in lower crust Melt-rock interactions in lower crust might be the most possible processes to produce these high-Mg and high-K calcalklic magmas. Hypabyssal rocks, mainly made up of granite porphyry and dioritic porphrite, show much higher ε Nd(T) than abyssal rocks. Granite porphyry are distinct with an adakite geochemical characteristics, high (La/Yb)_N, Sr/Y and low Rb/Sr ratio. The adakitic granite porphyry indicates a new lower crust produced by underplating within plate. Ultramafic xenoliths had been found only in augite diorites and amphibole diorites. Field investigations show that ultramafic xenoliths in augite diorites had been inherited from amphibole diorites, so ultramafic xenoliths had been only entrained by hydrous dioritic magma. Ultramafic xenoliths are mainly made up of dunite and harzburgite, orthopyroxenite and bistangite are the lessor. Coarse olivines in dunite often show many chromite exsolution lamellae. Opx in orthopyroxenite often show dense chromite exsolution lamellae. The presence of exsolution minerals indicates that ultramafic xenoliths had cooled before entraining. Metasomatism phenomenons are popular in dunite and harzburgite xenoliths, including two kinds of assemblage, cpx+phlogapite and opx+pl. The first metasomatism assemblage indicates an ancient enrichment. Rb-Sr dating of xenoliths shows that the ancient enrichment happened in 223 ± 7Ma. The second metasomatism assemblage indicates a recent, relatively not deep melt-rock interaction, which might be related with the genesis of the high-Mg high-K calcalklic rocks. Mineralogy and geochemistry indicate that these ultramafic xenoliths might sample the crust-mante transition zone (or the base of lower crust, moho). Investigation of high-Mg intrusions and their ultramafic xenoliths in Laiwu indicate that the thinning processes of Sino-Korea Plate can be divided into two stages. The first stage is lithosphere mantle thinning with crust thickening by underplating in lower crust. The second stage is that the thickened lower crust delaminated into the underlying mantle.