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1、Vol. 42 No. 2 SCIENCE IN CHINA (Series D) April 1999 Postcollisional mantle-derived magmatism, underplating and implications for basement of the Junggar Basin* HAN Baofu (韩宝福 ）， HE Guoqi (何国琦 ） and WANG Shiguang (王式洸） (Departmentof Geology， Peking University, Beijing 100871， China) Received March 29
2、, 1998 Abstract The late Paleozoic postcollisional granitoids, mafic-ultramafic complexes, and volcanic rocks are extensively distributed around the Junggar Basin; they are generally characterized by positive values, implying that the magmas were mantle-derived and contaminated with crustal*material
3、s to some extents. The emplacement of man- tle-denved magmas and their differentiates in the upper crust is the expression of deep geological processes at shallow level, while much more mantle-derived magmas were underplated in the lower crust and the region near the crust-nian- tle boundary， being
4、component part of basement of the Junggar Basin. The postcollisional mafic-ultramafic complexes would not be generated by re-melting of residual oceanic crust, which was considered as the basement of the Junggar Basin, unless very high degrees of partial melting occurred. Even if old continental cru
5、st had been present before collision, it would have been strongly modified by the mantle-derived magma underplating. This interpretation is compatible with the existing geophysical data. Keywords： depleted mantle postcollisional magmatism, underplating( basement, Junggar Basin. The Junggar Basin, si
6、tuated between Tianshan and Altai mountains, is one of the largest oil- and gas-bearing basins in China. The nature of basement of the Junggar Basin has long been debated. It has been proposed that the basin is underlain either by an ancient massif, with Precambrian basement of continental crust。 ，
7、or by oceanic crust2,3. The existence of the contrary opinions is due to the facts that no basement rocks are exposed within the basin and that there are no rocks representative of the basement in the surroundings of the basin. In such a case, what basement of the basin is has to be speculated on th
8、e basis of geophysical data2, geological correlation between the Junggar, Tarim, and Kazakhstan , and the limited isotopic data from the granites of West Junggar3. It is striking that the problem concerning the nature of basement of the Junggar Basin is very important not only for the tectonic evolu
9、tion of the Junggar Basin and even North Xinjiang, but also for exploration and development of oil and gas within the basin. On the basis of postcollisional mantle-derived magmatic rocks exposed around the Junggar Basin, this paper will discuss the nature of basement of the Junggar Basin. 1 Nd isoto
10、pic characteristics of postcollisional mantle-derived magmatic rocks The Junggar Basin is surrounded by Paleozoic orogenic belts, in which develops a large volume of granitoids. The latest plate convergence or collision took place in early Carboniferous time4. Isotopic datings have yielded the ages
11、of mafic-ultramafic complexes, most granitoids, and some volcanic rocks in a range of 330 to 250 Ma, younger than the collision event. Therefore, they are the products of postcollisional magmatism. * Project supported by the National Natural Science Foundation of China (Grants Nos, 4900031 and 49272
12、103). 71994-2017 China Academic Journal Electronic Publishing House. All rights reserved, http:/ 114 SCIENCE IN CHINA (Series D) Vol. 42 The postcollisional granitoids around the Junggar Basin are commonly characterized by positive e：Nd() values, indicating that the depleted mantle component is very
13、 important in the magmas (table 1). Evidently, Nd isotope signatures of the povStcollisional granitoids are in sharp contrast with those of the granitoids from the Caledonides, Hercynides, and Himalayas as their negative eNd() values imply a recycled crustal origin10. Limited data on volcanic rocks
14、show the Nd ivSOtope signatures like the granitoids (table 1) , and the postcollisional mafic-ultramafic complexes generally have the ages and Nd isotope signatures very similar to the granitoids (table 1), In addition, previous research work has revealed the Pb isotope signatures of the postcollisi
15、onal granitoids and mafic-ultramafic complexes like the depleted mantle-5,6,8,15,18-23. 2 Depleted mantle sources for postcollisional magmatism As mentioned above, the postcollisional magmatic rocks around the Junggar Basin generally have positive Nd() values. Assume that there is a linear evolution
16、 of Nd isotopic compositions in the depleted mantle, whose present-day e： Nd( ) value is + 10, and e ( t ) value at 4.57 Ga when the earth formed was 0. Then, only the tonalite-trondjemite in East Junggar has the Nd( ) value (+ 9) very close to that of the MORB-like depleted mantle at that time, and
17、 the remaining magmatic rocks have relatively lowvalues. This is commonly interpreted as a result of mixing of the mantle-derived magmas with crustal materials, but the positive eNd( r) values imply that the depleted mantle component is predominant. If the ophiolites in the surroundings of the Jungg
18、ar Basin are taken into consideration, it is not difficult to find that the depleted mantle was heterogeneous in composition and had a great variation in the degrees of depletion when these remnants of the ancient oceanic crust were formed. Some ophiolites had the Nd( ) values apparently lower than
19、that of the MORB-like depleted mantle. In East Junggar, for example, an Sm-Nd isochron dating yielded an age of (479 27) Ma for the ultramafic rocks in the Aermantai ophio- lite belt， with Nd(Z)二 + 6. 824， and another Sm-Nd isochron age for the Tuziquan ultramafic massif is (561 土 41) Ma， its eNd(，
20、)二 + 6,125. In West Junggar， the basalts and gabbro from the Tabale ophiolite yielded Sm-Nd isochron ages of (447 土 56) Ma (2cr) and (489 土 53) Ma ( 2 a ) , with Nd() + 7.3 and +5.8, respectively*-26, and two Sm-Nd isochron ages for the Hongguleleng ophiolite are largely different, being (626 25) Ma
21、 (la) and (444 27) Ma (2a), Nd( = +8.4 and + 7,226,27， respectively. Obviously， the initial Nd isotopic compositions of the ophiolites may correspond to those of the depleted mantle, unless they were contaminated by crustal materials. By assuming a linear evolution of Nd isotopic compositions in the
22、 depleted man- tie and Nd( = at 4.57 Ga, it i.s speculated on the basis of Nd( ) values of the ophiolites that the present-day ) values of the local depleted mantle as represented by the ophiolites are in a range of +6.5 to +10, and the (0 values at 300 Ma would vary in a range of +6.1 to + 9.3. The
23、refore, the postcollisional magmatic rocks with the (?) values intermediate between + 6.5 and + 10 of the local depleted mantle may not have been contaminated by crustal materials (fig. 1), and the variation in their Nd() values would reflect the mantle heterogeneity- The postcollisional granitoids
24、and mafic-ultramafic complexes have similar Nd and Pb isotope signatures, implying that they should have been derived from an identical source region. However, two contrary opinions are present in literature： either the postcollisional granitoids were generated by re-melting of the residual oceanic
25、crust with Paleozoic age-3- or they were mantle-derived 71994-2017 China Academic Journal Electronic Publishing House. All rights reserved, http:/ No 2 UNDERPLAlINCr AND BASP： MEN1( )F THE JUNG(AR BASIN 115 12,13 Aiatao Mountain Kongwusavi moyite 290 Wuiasitan granodiontc 290 Zutuhong adamellite 290
26、 Kazibieke alkali-feldspar granite 290 Chaganhundi alkali-feldspar granite 290 West Junggar Puerkesidai diabase-porphvrite 329 Buerkesidai alkaK-feldspar granite 314 Kelamav East alkali granite 321 Tianshan + 2.80 + 2 76 + 3.14 + 2 17 _ 1.88 2.43 Huangshan mafic-ultKimafic complex Huangshan East nia
27、iic-ultramafic complex Qiaoletiekexi gabbro Qiongawuzi gabbro Sulu gabbro Qingbulake olivine gabbro Qingbulake pvroxenite Kaldala olivine gabbro Qiulaketeqinke diabase Nileke quartz-albiU porphyry Kangguer rhyolite Kangguer tonalite Kumishi ostorogemc granite Kumishi granodiontc Kuerle area granite
28、309 + 6.60 1-6.69 5 320 + 7 71 + 7.88 L5 324 + 3 68 + 3 78 16 314 -4 93 - 5.07 16 322 + 6.40 16 320 + 3.79 16 320 + 3.87 16 320 + 6 00 17 320 + 1.68 17 248 + 0 79 + 0 93 5 300 + 0.22 + 0.61 5 275 + 0 31 + 3 59 5 334, 2 I 0 + 4 til 345 + 5 11 post Early -8 11 Carboniferous Table 1 Ages and (/) values
29、 of postcolksional magmatic rocks around the Junggar Basin Locality Lithology Age/Ma Nd ( ) Reference Altai Mountain Ashele rhyolite porphyry 294 + 3. 57 + 4 35 5 Ashele dacite porphyry 2% + 2. 50 + 4 53 5 Qicmaiqieke gneissic biotite granite 2()0 - 1 98 + 0 80 6 W uqiliketawu gneissic biotite grani
30、te 290 - 1 39 6 Keketuohai North gneisHic biotite granite 330 + 0 45 6 Tabieqi gneissic biotite granite 2)0 + 1.33 6 Alaayijuehuo quartz dionte 2l0 -0 55 6 Jlamukai (Daqiao) gneissic biotite granite 2)0 -5 34 6 Jiangjunshan aniazonite granite 230 + 】 6 + 2.0 6,7 East Junggar Kalatongke mafic-ultrama
31、fic complex 298 + 5 94 - 6 (38 5 Saerbutake dionte 20 + 5 18 8 Ruergen granodiorite 2W + 3.42 8 Buergen movitc 250 + 4.1。 8 Buergen nebeckite granite 250 + 0 64 8 Ertai alkali gabbro 20 + 7 27 8 Ertai alkali syenite 290 -3.81 8 J iedekala alkali granite 300 + 6 09 + 6 67 9 Sawudeger alkali granite 3
32、0U + 5 56 + 6.47 9 Saertietieke alkali granite 300 + 5 12 + 5 61 9 Tasigake alkali granite 300 + 5 06 + 5.54 9 Yebushan alkali-feldspar granite 268 -+ 5 51 + 5 84 10 tonalitc-Trundjemite 316 + 9 11 granodiorite-granite 300 + 7 11 2222 445 71994-2017 China Academic Journal Electronic Publishing House
33、. All rights reserved, http:/ 116 SCIENCE IN CHINA (Series D) V l. 42 magmas produced by partial melting and their differentiates28-. If the post colli- sional granitoids were produced by re- melting of the residual oceanic crust, then the comagmatic mafic-ultramafic complexes would be produced only
34、 by high degrees of partial melting, and the oceanic crust should be re-melted in a short-time span after its formation. In this case, it seems very difficult to gener- ate large amounts of postcollisional grani- 220 240 260 280 300 320 340 細 ,仏 toids and mafic-ultramafic complexes. Therefore, it is
35、 speculated that the mafic- Fig. 1. ewriC?) vs. intrusive age diagram for the postcollisional man- i. / 一 i i, j r 6 ultramaiic complexes resulted from the tie-derived magmatic rocks around the Junggar Basin. The range for lo- - . ., A ！ ,. T . . r i , emplacement ol the magmas generated by cal depl
36、eted mantle is based on Nd isotopic data irom the ophiohtes vsee 1 text for details). partial melting of the local depleted man tle in the upper crust, and the postcollisional granitoids were the products of the residual mantle-derived magmas after strong differentia- tion28 . 3 Underplating and bas
37、ement of the Junggar Basin Underplating is a process of emplacement of the magmas derived from deep mantle in the crust. During the postcollisional stage of an orogenic process, underplating may be very important, because such a tectonic setting is favorable to the emplacement of mantle-derived magm
38、as in the crust. If mantle-derived magmatism results in underplating at or near the crust-mantle boundary and in the lower crust, relevant volcanism occurs at the surface, and small intrusions intrude into the upper crust29,30, while large amounts of mantle-derived magmas would be underplated due to
39、 high density. Even if underplating is accompanied by a large-scale flood volcanism at the surface, only 30% magmas can reachthe surface, and the remaining would be mainly emplaced in the lower crust31 The postcollisional granitoids and mafic-ultramafic complexes around the Junggar Basin occur as sm
40、all intrusions, and they are predominated by granitoids and evidently have isotopic characteristics of mantle-derived magmas and their differentiates. This implies that only very small amounts of magmas were emplaced in the upper crust, leading to the formation of the mafic-ultramafic complexes, and
41、 the granitoids were highly evolved differentiates of mantle- derived magmas. Accordingly, large amounts of magmas may be emplaced in the lower crust and at or near the crust-mantle boundary. The lower crust produced by underplating is mafic in composition and is generally character、 ized by high se
42、ismic velocity and density. The previous discussions on basement of the Junggar Basin have been based upon the interpretation of geophysical data1,2 The extensive development of the postcollisional granitoids and mafic-ultramafic complexes around the Junggar Basin indicates that the postcollisional
43、mantle-derived magmatism was intense and that it had widespread expression at the surface. During the postcollisional mantle-derived magmatism, the magmas were emplaced not only in the upper crust but also in the lower crust and in the region near the crust-man- 71994-2017 China Academic Journal Ele
44、ctronic Publishing House. All rights reserved, http:/ No. 2 UNDERPLATING AND BASEMENT OF THE JUNGGAR BASIN 117 tie boundary. The mantle-derived magma underplating would make the basement of the Junggar Basin change greatly, resulting in the fact that the deep crust shows the geophysical characterist
45、ics such as high gravity and magnetic anomalies1,2 and high seismic velocity32. The mafic rocks produced by underplating should show pronounced differences in either geochemical or geoephysical features. If the basement of the Junggar Basin was oceanic crust, its geophysical features should complete
46、ly differ form continental crust32 . Thelowercrustofmafic-ul- tramafic volcanic r cks2 or mafic blocks32 beneath the Junggar Basin may be the products of the mantle-derived magma underplating. 4 A tectonic model The distribution of the postcollisional mantle-derived granitoids around the Junggar Bas
47、in is principally controlled by major fault zones, so they have been called k*deep fault zone-con trolled granites”6. The distribution of the mantle-derived granitoids in East and West Junggar are spatially parallel to the ophiolite zones, implying that major fault zones may have had an important ro
48、le in controlling the ascendance and emplacement of mantle-derived magmas and their differentiates. In addition, the change in tectonic regime from synorogenic compression to postorogenic extension would be favorable to the emplacement of mantle-derived magmas and their differentiates in the upper crust. After collision, portion of the thickened lithospheric mantle would be delaminated, accompanied by hot asthenospheric mantle upwelling. At a relatively shallow level, partial melting of the mantle would take place due to decompression, the mantle-derived magmas ascend t