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PETROGENESIS OF LATE MESOZOIC GRANITIC VOLCANIC-INTRUSIVE COMPLEXES IN SOUTHEASTERN CHINA

 

WANG Dezi ,  ZHOU Jincheng  and  QIU Jiansheng

Department of Earth Science, Nanjing University, Nanjing  210093, China

 

There occurred intensive magmatism in southeastern China during late Mesozoic. A typical feature is that acidic rocksgranites and rhyolitespredominate in the area, accounting for 95% of total igneous rocks, while andesites and basalts only make up less than 5%. The time-span of the volcano-intrusive activities has been defined as middle Jurassic to late Cretaceous and shows a tendency of getting younger from inland to coastal area. The width of magma arc is up to 600 km. The granites are spatially associated with coeval rhyolites and dacites. From coastal area to inland, volcanic rocks become less and less. They almost disappear near the Ganjiang River in Jiangxi Province. There only occur granites in a vast area to the west of the Ganjiang River. The Ganjiang fault therefore may be considered as the western border of late Mesozoic volcanic rocks in southeastern China and named as volcanic line.

The subvolcanic granitoids, i.e. hypabyssal granitoids closely related to acidic and intermediate-acidic volcanic rocks, are widely distributed in SE China. They often emplaced within calderas. Based on studies of many instances, it is noted that only a small amount of the magma in a magma chamber could erupt onto the surface, while most magma would remain underground to form various sized granitic intrusive bodies at different levels. These granite bodies may possibly be considered as the root of central type volcanic apparatus composed of rhyolites and dacites. They together are called granitic volcanic-intrusive complex. As a geochemical criterion, these granitic volcanic-intrusive complexes show some common features in emplaced time, space and source materials. Temporally, the eruption and emplacement of acidic vocanics and granites occurred almost simultaneously. Spatially, the acidic volcanics and granites are associated in the same or nearby volcanic apparatus. Finally, the acidic volcanics and granites from the same complex were originated from similar source materials. For better understanding the petrogenesis of late Mesozoic granitic volcanic-intrusive complex in SE China, it is significantly helpful to analyse the features of emplaced time, space and source materials of  some typical complexes in the area.

Tonglu volcanic-intrusive complex, located in a fault-depression basin with an area of about 460 km2, is a typical example of I-type granitic volcanic-intrusive complex. The volcanic-intrusive complex in the basin was formed in three stages. At the first stage, rhyolitic magma was erupted along the border of the basin, forming rhyolitic tuff and ignimbrite. At the second stage, rhyodacitic magma had been extruded out along the middle fault in the basin, forming the dome composed of clastoporphyritic lava. At the last stage, quartz dioritic magma had been intruded into the dome, forming hypabyssal intrusive rocks with subvolcanic characteristics. Their single-zircon U-Pb dates are as follows: rhyolitic volcanics, 134.9 Ma; quartz monzonitic diorite , 134.4 Ma. It illustrates that three phases of the complex in the basin were contemporaneously formed (Fig. 1). Moreover, the rocks formed in three stages have similar Rb-Sr and Sm-Nd isotopic initial values: (87Sr / 86 Sr)i = 0.708, eNdt= -5.27 for rhyolitic volcanics formed in the first stage; (87Sr / 86 Sr)i =0.708, eNdt= -5.37 -6.09 for rhyodacitic volcanics in the second stage; (87Sr / 86 Sr)i = 0.708, eNdt= -5.91 -6.15 for quartz monzonitic diorite in the third stage. There is no obvious difference in the abundance of REE and other trace elements of rocks in three phases, suggesting these volcanic, subvolcanic and hypabyssal rocks are undoubtedly comagmgtic. Both the temporal and spatial distributions of the Tonglu volcanic-intrusive complex indicate that there exists a zoning magma chamber in shallow level under the volcanic apparatus. The magmas reserved in the zoning chamber include, from top to bottom, rhyolitic, rhyodacitic and quartz dioritic. The mineral association and lithogeochemical data of the volcanic-intrusive complexes show that the fractional crystallization of plagioclase and hornblende in quartz dioritic magma led to the formation of the compositional zoning in the hypabyssal magma chamber.

 

Fig.1.  Sketch geological map of the Tonglu volcanic basin

 

1. Pre-Jurassic strata; 2. Rhyolitic tuff ; 3. Quaternary; 4. Rhyolitic ignimbrite; 5. Quartz monzonitic diorite;

6. Clastoporphyritic rhyodacite(extruded phase); 7. Monzonitic granite porphyry (subvolcanic phase);

8. Crater; 9. Fault; 10. Stratigraphic boundary.

 

The S-type granitic volcanic-intrusive complex in China was first discovered at Xiangshan, Jiangxi Province. There is a large-scale collapse volcanic basin at Xiangshan. Volcanic activity in this basin can be clearly divided into two cycles. The first cycle volcanism displayed as fissure eruption along a EW-trending fracture and formed intermediate-acidic rhyolite and dacite; whereas the second cycle occurred as central type eruption and formed porphyroclastic rhyolite which contains aluminum-rich minerals, such as andalusite and garnet. Zircon U-Pb dating demonstrates that the porphyroclastic lava erupted at 140 Ma. After intensive volcanic eruption, a circum-basin volcanic collapse structure was formed. The subvolcanic rocks intruded along the circum-basin fractures and formed ring dykes of sub-granite porphyry. Zircon U-Pb age of the sub-granite porphyry is 135 Ma,  only 5 Ma less than that of the porphyrocalstic lava, thus both were formed almost at the same time. The basement of this volcanic apparatus is Proterozoic metasedimentary rocks, whose lead isotopic compositions are similar to that of the volcanic-intrusive complexes. The (87Sr/86Sr)i ratios of the sub-granite porphyry vary between 0.71096 and 0.71201, eNd(t) values range from C11.2 to C7.8, and Nd model ages change from 1559 Ma to 1865 Ma. The corresponding values of the porphyroclastic rhyolite and rhyolitic dacite are 0.7110.712, C9.4 C8.4 and 16421697 Ma respectively, which are close to that of the sub-granite porphyry. Therefore, we believe that the granitic magma of the volcanic-intrusive complex at Xiangshan was formed by partial-melting of the Proterozoic metasedimentary basement and can be ascribed to S-type on genetic aspect (Fig. 2).

 

Fig.2.  Sketch geological map of the Xiangshan volcanic-intrusive complexes

1.gravel; 2. porphyroclastic rhyolite; 3. rhyolitic dacite; 4. sandstone, gravel; 5. sandstone;

6. metamorphic rocks; 7. sub-granite porphyry; 8. sub-granodiorite porphyry; 9. granite;

10. volcanic neck (inferred); 11. fault

 

Typical A-type granites were developed at Kuiqi, Fujian Province, and representative A-type rhyolites were discovered at Yunshan, Fujian. Both Kuiqi and Yunshan were located within the same fault zone, about 60 km apart. The Rb-Sr isochron dates of the A-type rhyolites at Yunshan and A-type granites at Kuiqi are 104 Ma and 93 Ma respectively, showing similar forming ages. Their lithogeochemical features are also very similar. Both are characterized by containing alkaline mafic minerals. They are rich in silicon and alkali, low in aluminum and calcium, and show seagull-type REE distribution patterns (Fig. 3). On trace element aspect, both of them are enriched in LILE (e. g., Rb, Th) and HFSE (e. g., Nb, Zr) with high Ga/Al ratios and low Sr, Ba (Fig. 4). The Nd isotopic compositions of Yunshan A-type rhyolites and Kuiqi A-type granites are quite similar, with eNd(t) values equal C3.62 C4.29 and C3.45C5.21 respectively, indicating that they have similar magma source. Therefore, the Yunshan A-type rhyolites and the Kuiqi A-type granites can be undoubtedly classified as the A-type granitic volcanic-intrusive complex.

 

Fig.3.  Chondrite-normalized REE distribution patterns of A-type rhyolites at Yunshan

(labeled as Ys- or PI-) and A-type granites at Kuiqi (labeled as Kq)

 

Fig.4.  Primordial mantle-normalized distribution patterns for trace elements of A-type rhyolites at

Yunshan (labeled as YS-) and A-type granites at Kuiqi (labeled as Kq)

Now we can confirm that the late Mesozoic granitic volcanic-intrusive complexes in SE China can be divided into three types, that is, I-type, S-type and A-type. Their formation is closely related to the subduction of the Pacific plate beneath the continental block of SE China and the induced underplating of basaltic magma. To reveal the genetic mechanism of granitic volcanic-intrusive complexes, it is necessary to study the heat source which resulted in large-scale melting of crustal materials in addition to investigating their material source. Some basic granulite enclaves have been found at Cenozoic basaltic breccia pipe in SE China, and the ages of these enclaves (112 Ma) are corresponded to that of the surrounding granites and volcanic rocks. By integrating the above fact with the related geophysical data, it has been confirmed that there exactly exists underplating of basaltic magma in SE China. This process supplies heat and various amount of mantle materials for the formation of granitic volcanic-intrusive complexes.

The spatial distribution of the different type granitic volcanic-intrusive complexes displays a clear regularity. In general, the I-type granitic volcanic-intrusive complexes distribute mainly along the coastal area of SE China, whereas the S-type occurred predominantly in inland to the west of the Wuyi Mountain. This distribution pattern was controlled mainly by two factors, one is the compositions of basement rocks, and the other is the intensity of crustal-mantle interaction. During late Mesozoic, the underplating of basaltic magma in the coastal area of SE China was more active than that in inland, the crust-mantle interaction may also be more intensive in the coastal area. To the west of the Wuyi Mountain, the continental crust shows higher degree maturity, and the basement is composed predominantly of metasedimentary rocks. These factors are favourable for the formation of S-type granitic volcanic-intrusive complexes. A-type Granitic volcanic-intrusive complexes occurred mainly in the coastal area of Zhejiang and Fujian Provinces, and their distribution is controlled obviously by the Changle-Nanao deep fault. With an upwelling of asthenosphere, the thickness of lithosphere and crust in these areas were clearly thinned. A-type granitic volcanic-intrusive complexes were just formed under such an extensional tectonic setting. Spatially, the A-type granitic volcanic-intrusive complexes are accompanied with late Mesozoic bimodal volcanic rocks, thus forming a typical extensional tectonomagmatic association in continental margins. The occurrence of this rock association indicates that the Yanshanian tectonic-magmatism in SE China was close to the end at that time. Afterwards, the granitic magmatism was followed by large-scale Cenozoic basaltic magmatism.

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