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The discovery and significance of the northeastern Jiangxi Province ophiolite (NEJXO), its metamorphic peridotite and associated high temperature-high pressure metamorphic rocks
Journal of Southeast Asian Earth Sciences, Vol. 3, Nos 1-4, pp. 237-247, 1989
0743-9547/89 $3.00 + 0.00 Maxwell Pergamon Macmillan pie
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The discovery and significance of the northeastern Jiangxi Province ophiolite (NEJXO), its metamorphic peridotite and associated high temperature-high pressure metamorphic rocks ZHOU GUOQING Department of Earth Sciences, Nanjing University, Nanjing, People's Republic of China Abstract--The NEJXO with a N.E.-S.W. elongation occurs in the mid-Lower Qigong Group, under which lies the Jiuling Group (1401 Ma) and above which lies the Shangshu Group (817 + 87 Ma), so that the age of NEJXO is defined to be Proterozoic between 1401 Ma and 817 + 87Ma. The sediments of the Jiuling Group show evidence of continental derivation, but the Qigong Group and Shangshu Group are characterised by CA volcanic rocks and probably represent a gradually growing island-arc. Thus, we regard the NEJXO as occurring in a back-island-arc basin between the ancient continent and the island-arc. On the whole, the main members of dismembered ophiolite are all present. The metamorphic periodotite present in them, is considered to be especially important, because it may be the sole representative of the older mantle present and it differs from those younger. The high-T metamorphic rocks associated with the NEJXO are various hornstones and melilite marble, whereas the high-P metamorphic rocks are aragonite-jadeite--glaucophane schist and schistose lawsonite marble. From the fact that high-P metamorphism was superimposed on the high-T metamorphic rocks, it may be suggested that early tension (at opening stage) and late compression (at closing stage) occurred during the development of the basin.
INTRODUCTION Tim NORTHEASTERNregion of Jiangxi Province adjacent to Anhui and Zhejiang Provinces, was known as the northeastern margin of the "Jiangnan earth axis" (Huang et al. 1965), the "Jiangnan Dongan-Xuefeng island-arc folded system" (Guo et al. 1980) and the "Yangzhi paraplatform" (Ren et al. 1980). In this area, more than 100 basic and ultrabasic bodies outcrop from Yiyang, Dexing, Wuyuan in Jiangxi to Xixan in Anhui in a S.W.-N.E. direction. Zhu et al. (1983) suggested that these bodies formed at the Yanshan Stage and marked the "N.E. Jiangxi Deep Fault Zone". At the First Chinese Basic, Ultrabasic Rocks and Ophiolite Symposium, 1981, I first proposed that those basic and ultrabasic bodies occurring in N.E. Jiangxi together with the Proterozoic meta-volcanic rocks represent a regular ophiolite suite and named this the "northeastern Jiangxi ophiolite" (NEJXO) (Fig. 1).
THE PROOF OF NEJXO At the Penrose Meeting, 1972, an ophiolite suite was defined as an association of a specific rock series representing development of the oceanic crust (Coleman 1977, Gass 1982). The evidence for interpreting the occurrence of an ophiolite suite in N.E. Jiangxi is as follows: 1. As a whole, the main members of an ophiolite suite are present, such as metamorphic peridotite, cumulates, diabase and metalavas (as well as silicalites), etc., but in places one or two of them may be absent as a result of dismembering; 2. The presence of fragments of members mentioned above can be found in some metabreccias and volcanic
breccias, which indicates that before some metamorphic event during the Proterozoic, there was already a close relation between those members; 3. The members always occur together along the N.E.-S.W. belt in space; 4. Among the members, there is an intimate relation in the composition and origin, which will be demonstrated in the following sections.
THE OCCURRENCE AND AGE OF NEJXO Based on the following reasons, I believe that the NEJXO developed in the mid-late Proterozoic: 1. The NEJXO was cut through the middle by the Damaoshan Granite which has been determined to be 135 Ma in age (K-Ar, biotite, 1974); also, it intruded into the Triassic-Jurassic Anyuan Group and an ultrabasic body. Therefore the NEJXO is older than the Yanshan Period (about 100 Ma); 2. We cannot see any contact between the NEJXO and the Sinian, Cambrian or Ordovician strata, but there is a tectonic contact between the NEJXO and the Permian, Triassic as well as Jurassic. However, the NEJXO always occurs within the Proterozoic epirocks and fragments of its members are associated together in metabreccias and metavolcanic breccias; 3. The schistosity that developed within phyllite passed through the serpentinite; 4. The NEJXO lies in the mid-Lower Qigong Group which overlies the Jiuling Group and underlies the Sanshu Group. The Sanshu contains gravels of the serpentinite in the basal conglomerate. According to the Bureau of Geology and Mineral Resources of Jiangxi
237
238
ZHOU GUOQrNG the NEJXO must have occurred in a setting related to an island-arc.
I
PETROLOGY AND MINERALOGY OF NEJXO
Is there any difference between Proterozoic and postProterozoic ophiolites? To this question the NEJXO may be able to provide some answers. I believe that the Proterozoic metamorphic peridotite is especially worthy of further attention.
1. Metamorphic peridotites
I I I JlK/'"L'~"uu"uuu"
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Fig. I. Geological sketch map of the northeastern Jiangxi ophiolite suite. 1. Jiuling Group; 2. Qigong Group; 3. Sinian~)rdovician system; 4. Mesozoic volcanic rocks; 5. Mesozoic basin; 6. Yanshanian granites; 7. Xuefengian granites; 8. Ultrabasic bodies representing NEJXO, plus high-temperature metamorphic rocks (T) and high-pressure metamorphic rocks (P); 9. Main faults.
Province (1984) and Shu and Li (1987), a Rb-Sr isochron age of tuffaceous phyllite in the lower Shuangqiac~han Group (corresponding to the Jiuling Group) is 1401 Ma old, and a Rb--Sr age for metarhyolite in the top of the Shanshu Group is 817 _+ 87 Ma old. Xu Bei (personal communication), obtains a Sm-Nd isochron age of 915 _ 34 Ma for the NEJXO. From the above it is clear that the age of the NEJXO is about 900-1000 Ma. Care must be taken when applying petrochemistry and geochemistry to distinguish the tectonic setting of a ophiolite, and a detailed geological analysis is required, especially for old, dismembered and metamorphosed ophiolite. Recently, Shu and Li (1987) proposed that the NEJXO melange marks a terrane collage zone between the Jiuling Terrane and the Huaiyu Terrane and the zone coincides with Zhu's (1983) N.E. Jiangxi Deep Fault Zone (Fig. 1). The Jiuling Terrane is mainly made up of the Jiuling Group, which contains epimetamorphic sandstone, siltstone, pelite and meta intermediate-basic volcanic rocks at the top. These sediments come from the north, and are of continental provenance. The Huaiyu Terrane consists of the Qigong Group and the Sangshu Group. The Qigong Group contains spilite-keratophyre series, metabasalt-andesite series and abundant pyroclastics, and the Shangshu Group mainly contains a calc-alkaline series made of basalt, andesite and rhyolite. Both these groups appear to represent a gradually matured island-arc, therefore
Metamorphic peridotites occur at Xiwan, Dazishan and Maoqiao, etc., and consist mainly of harzburgite, dunite (Xiwan, Dazishan), olivine-websterite and augite-peridotite (Maoqiao). They are fresher than the cumulate ultramafic rocks which have been serpentinised. In Xiwan, abundant inclusions of harzburgite are included in the serpentinised cumulate ultramafic rocks which is one of the obvious differences. The foliations of the harzburgite inclusions with hobnail strike at 90°-270 °, 45°-225 ° and 30°-210 °, but the schistosity of the serpentinised cumulate ultramafic rocks strike along 180°-360 °. The olivines of the dunite at Maoqiao appear with kink-bands. The refractive indices of the olivine measured by the immersion method are Np = 1.6520, Nm = 1.6659, Ng = 1.6827, Ng-Np = 0.0293, (+)2V = 84.5, Fo = 93 __+1 (sample R80). The electron microprobe data and calculated formulae of the olivine (Fo: 89.17) from sample Mao 191 are listed in Table 1. Table 1. Microprobe analyses and calculated formulae for olivine, enstatite and chrome spinel in dunite (Mao 191)
SiO2 TiO2 A1203 Cr203 NiO MgO FeO* MnO CaO Na20 K20 Total
Olivine
Enstatite
Chrome spinel
38.25 0.04 0.20 0.09 0.44 50.33 10.53 0.11 -0.07 -100.06
54.07 0.04 2.00 0.13 0.08 35.92 7.32 0.15 0.16 0.17 -100.04
0.22 0.39 23.32 39.51 0.14 12.12 23.98 0.23 0.03 0.09 -100.03
Mineral formulae Si 0.935 Ti 0.001 AI -Cr 0.001 Fe ~+ -Fe 2+ 0.215 Ni 0.009 Mg 1.835 Mn 0.002 Ca -Na 0.002 K
Total Oxygens
- -
3.000 4
1.889 0.001 0.004 0.002 -0.214 0.002 1.871 0.004 0.006 0.006 - -
4.000 6
0.001 0.009 0.884 0.959 0.276 0.433 0.004 0.555 0.006 0.001 0.006 - -
3.134 4
Features of northeastern Jiangxi ophiolite (NEJXO) Along fractures in the dunite, olivines have been transformed into clusters of enstatite (Fig. 2). The microprobe data and calculated formulae of the enstatite from Mao 191 are also listed in Table 1. After the formation of the enstatite, some diopside, phlogopite and carbonates are formed in succession. The last mineral to form is serpentine. From the above evidence it can be suggested that after the dunite was fractured, it had experienced "dry" heating followed by "wet" chemical alteration. Considering only heating and neglecting others, the transformation of olivine to enstatite should occur at 1557°C, if considering pressure and other factors as well this transformation temperature must be higher. The composition and calculation of chrome spinel in Mao 191 can be seen in Table 1. A special and rare characteristic "grated" texture (Fig. 3) present in the dunite is very interesting and differs from that shown by meteorites. It consists nearly all of olivine bars probably caused by cross-gliding. The harzburgite occurring at Xiwan has a protogranular texture, the kink bands of the olivines are clear and common, and sometimes display fan-shaped bands (Fig. 4). In this case, the boundaries between the kink bands are not as obvious as those in younger harzburgites and it appears they had undergone another phase of plastic deformation. This is another important feature of the Proterozoic metamorphic peridotite. In the enstatite, kink bands, undulatory extinction, glide-twins and exsolution lamellae can be seen. The ovoid olivine or diopside may be encircled by enstatites, which suggests that magmatism had occurred before the deformation of the metamorphic peridotite. The lherzolite-olivine websterite is fresh, coarsegrained and is made up of olivine, enstatite, diopside and chrome spinel. Since the percentage of minerals in them is variable, the rocks form a series from lherzolite to olivine-websterite. Kink bands, exsolution lamellae, two directional glides and glide twins in both clinopyroxenes and orthopyroxenes can be seen. As the exolution lamellae are kinked, it is suggested that the exsolution had been formed before kinking. Some enstatite exhibits an irregularly clear rim, but some olivines are grain recrystallized from fine olivines crushed from some earlier coarse olivines. Coleman (1977) defined a metamorphic peridotite as a peridotite with tectonic fabric, mainly harzburgite, representing a mantle material that has undergone a subsolidus rheomorphism rather than chemistry. The metamorphic peridotite without cumulate fabric in the area fits into this definition. 2. Cumulative rocks
Cumulate rocks present include augite-peridotite, chromitite, gabbro-diorite and plagioclasite. The rocks commonly show intense alteration, in which the augite peridotite have nearly all been transformed into serpentine. Among the cumulate rocks, the augite--peridotite is dominant, others are subordinate. All these rocks
239
exhibit cumulate texture but bandstructure is not developed. These features suggest that the crust was thinner at that time and the magma was derived from the mantle close by. This magma was more ultrabasic and more active. Diabase is often present as in tectonic blocks mixed with serpentinite and sometimes occur as individual dykes intruding into phyllite. Sheeted dyke swarms have not been seen. 3. Lavas
The lavas are represented by the spilite-keratophyre series and the basalt-andesite series. The spilitekeratophyre rocks have been altered into "green stone series". It is significant that we have not yet found any pillow lava. However, volcanic pyroclastic rocks accompanying the lavas are highly developed. Other members within the NEJXO are the tuffaceous phyllite, zoned silicalites, siliceous shales, red jasper, calc-shales and limestone.
THE PETROCHEMISTRY AND GEOCHEMISTRY OF NEJXO
Bulk rock chemical analyses and analysis of the crossite were done by the Analysis Center of the Department of Earth Sciences, Nanjing University. Chemical analyses of other minerals were carried out on a JEOL JXA-733 Electron Microprobe at this Maanshan Mining Research Institute, Ministry of Metallurgical Industry, and a Hitachi X650 Scanning Electron Microanalyzer with EDAX 9100 in the Center of Material Analysis, Nanjing University. Bulk rock REE analyses were made by a GP3.5-E Inductively Coupled Plasma Quantometer in the Yichang Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences. We should note that the mantle is not invariant and that the ophiolitic metamorphic peridotites, one of mantle material, and other related ophiolites are also not invariant. Thus, the petrochemistry and geochemistry of ophiolites of various ages must have in addition to their general characters, inherited characters and individuality and variability. This information can be obtained by comparing the Proterozoic ophiolite with those from the Meso-Cenozoic era, but this comparison ought not to be mechanical. On the other hand, the Proterozoic metamorphic peridotite may more approach the protomantle and is therefore an important source for information regarding the old mantle to which we must pay great attention. We shall emphasise the petrochemistry and geochemistry of the metamorphic peridotite within NEJXO. A comparison of the metamorphic peridotites of Proterozoic ophiolite with those occurring mainly in younger ophiolites, can be made by using Coleman's (1977) AFM, ACM, and SiO2-100 x FeO*/(FeO + MgO) diagrams, and similarly comparison between the metamorphic peridotites and cumulates can be made. Results
2
3
4
0.80
0.802 2.768 0.464 1.715 0.483 0.207 0.469 0.085 0.462 0.078 0.227 0.038 0.202 0.016 1.417
0.82
0.409 0.439 0.166 0.214 0.095 0.061 0.072 0.035 0.062 0.020 0.042 0.083 0.018 0.007 0.173
0.469 1.058 0.381 1.543 0.405 0.216 0.477 0.081 0.436 0.083 0.245 0.036 0.237 0.018 1.339
0.78
1.109 1.547 0.372 0.943 0.198 0.044 0.130 0.044 0.122 0.031 0.081 0.025 0.050 0.010 0.528
0.83
5
6
7
8
0.836 2.605 0.306 0.660 0.172 0.066 0.095 0.037 0.085 0.020 0.050 0.018 0.023 0.059 0.276
0.77
0.853 2.117 0.348 0.857 0.285 0.155 0.364 0.085 0.541 0.105 0.350 0.057 0.474 0.036 1.969
0.87 1.279 3.826 0.488 1.458 0.492 0.173 0.807 0.162 i.046 0.218 0.761 0.096 0.492 0.064 4.409
0.80
0.84
39.66 49.08 48.52 40.85 0.151 0.057 0.302 0.220 3.49 0.90 4.91 1.57 5.88 2.28 1.64 6.63 4.58 2.56 2.85 0.87 0.139 0.155 0.124 0.050 32.63 3 1 . 3 1 1 7 . 6 3 37.16 3.40 5.52 20.95 -0.25 0.04 0.24 -0.04 0.005 <0.005 0.08 0.031 0.017 0 . 0 3 1 0.960 9.44 8.34 2.04 12.37 99.69 100.26 99.24 100.76
9
!.109 1.303 0.414 0.815 0.405 0.060 0.217 0.102 0.148 0.057 0.131 0.057 0.114 0.013 1.260
0.81
33.88 0.057 1.16 3.70 4.47 0.098 33.38 0.40 0.16 0.07 0.017 21.62 99.01
10
0.938 0.977 0.563 1.286 0.345 0.081 0.373 0.119 0.305 0.081 0.184 0.057 0.088 0.018 1.496
0.78
39.36 0.069 1.68 7.69 3.42 0.072 35.74 0.03 0.02 0.005 0.017 12.44 100.54
11
0.85
40.78 0.080 0.82 6.98 0.50 0.060 37.55 0.38 0.08 --12.54 99.77
12
0.85
38.88 0.030 0.72 6.16 1.23 0.110 38.16 0.25 --0.01 13.96 99.51
13
14
1.364 1.628 1.076 1.543 0.655 0.147 0.746 0.179 1.655 0.148 0.577 0.105 0.667 0.026 5.591
0.72
0.49
51.84 52.30 0.245 0.950 15.37 18.38 1.09 1.78 3.03 4.57 0.124 0.12 10.25 5.99 8.70 9.66 4.40 2.89 0.16 0.45 0 . 0 3 1 0.07 4.22 3.00 99.46 100.16
15
16
17
18
19
20
4.434 6.838 1.986 6.602 1.380 0.466 1.822 0.298 2.004 0.367 1.312 0.158 0.834 0.132 11.811
0.46
0.38
3.837 4.775 7.327 5.780 2.400 2.152 6.516 8.403 1.639 2.070 0.544 0.734 1.909 2.776 0.391 0.434 2.701 2.701 0.455 0.620 1.662 2.011 0.228 0.245 1.405 !.317 0.185 0.202 13.386 18.898
0.49
0.49
0.26
0.29
53.60 52.60 51.30 53.03 46.21 51.08 0.866 0.797 1.220 1.010 1.980 0.650 16.15 15.89 14.60 14.36 13.29 13.44 1.77 2.13 3.95 1.35 5.27 3.54 5.34 5.26 6.70 6.90 10.48 4.36 0.139 0.129 0.201 0 . 2 1 0.23 0.42 5.95 6.90 6.50 7.82 6.12 10.64 8.80 9.75 7.85 10.20 9.20 9.05 3.46 2.84 2.86 2.62 2.49 2.50 0.80 0.58 0.29 0.28 0.13 0.58 0.129 0.112 0.129 0.17 0.28 0.08 2.12 2.30 3.48 1.98 4.12 3.06 99.12 99.29 99.08 99.93 99.80 99.40
21
0.59
52.83 0.990 14.63 1.96 6.12 0.15 7.77 9.70 2.89 0.29 0.23 2.34 99.90
22
23
0.50
0.27
55.16 41.81 0.580 4.290 17.45 14.30 2.10 5.05 5.95 13.48 0.27 0.19 5.12 6.62 5.11 9.31 2.92 1.21 1.06 0.19 0.17 1.08 3.94 3.22 99.83 100.75
1-4, metamorphic dunites; 5-6, augite peridotites; 7, olivine websterite; 8, harzburgite; 9-12, cumulate augite peridotites; 13-14, cumulate gabbros; 15-16, diorites; 17-18, diabases; 19-21, meta-basalts; 22, meta-andesite; 23, ultrabasic lava.
FeO+MgO La Ce Pr bid Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y
MgO
CaO Na20 K20 P205 LOI Total
MgO
TiO2 A1203 Fe203 FeO MnO
$iOz
1
39.14 45.12 44.96 40.34 0.044 0.076 0.126 0.025 0.90 1.03 i.42 0.64 6.27 4.07 5.35 5.49 2.60 4.11 3.96 2.27 0.098 0.139 0.160 0.217 37.44 31.78 30.55 34.89 0.I0 4.15 4.05 2.33 <0.01 0.11 0.07 0.02 <0.005 0.01 0.01 0.005 0.031 0 . 0 3 1 0.031 0.017 13.10 8.84 8.80 12.66 99.74 99.47 99.77 98.90
Table 2. Chemical analyses and REE composition (ppm) for NEJXO
N
bo 4~
Fig. 2. The clusters of the enstatites transformed from olivines in dunite from Maoqiao. Crossed polars, 1 bar = 0.2 mm. En, Enstatite; Tr, Tremolite.
Fig. 3. The "grated texture" composed of crossed olivine bars in dunite from Maoqiao. Crossed polars, 1 bar = 0.2 mm. O1, olivine.
Fig. 4. The fan-shaped kink bands of the olivines in the harzburgite from Xiwan. Crossed polars, 1 bar = 0.2 mm. O1, olivine.
241
Fig. 8. The melilite marble from Zhangshudun. Crossed polars, 1 bar = 0.2 mm. Mel, melilite: Cc, calcite
Fig. 9. The glaucophane schist from Xiwan. Crossed polars, 1 bar = 0.2 ram. GI, glaucophane (crossite).
242
Features of northeastern Jiangxi ophiolite (NEJXO) At
/ CaO
0
243
FeO
K°maTe'7 ' .(!i!.!t!!. i i ~ i ~L i e t' ~ ' ' Na20* Kz0
MgO
Fig. 5. (a) AFM diagram comparing the metamorphic peridotite, maflc and ultramafic rocks from NEJXO with Coleman's metamorphic peridotites, marie and ultramaric cumulates (1977). (b) Triangular diagram of MgO-CaO-A1203 comparing NEJXO containing metamorphic peridotite with Coleman's (I 977), Nicolas' and Jackson's (1972) metamorphic peridotites. (a) The distribution range of dunite and harzburgite, (b) the distribution of iherzolite. • NEJXO; the numbers are the same as in Table 2.
are given in Table 2, and plotted in Figs 5 and 6. It can be seen that both the metamorphic peridotite and ultramafic cumulates plot in Coleman's "metamorphic peridotite area" and "cumulate area", or even outside these two areas (e.g. no. 7). It appears rather unsuitable to differentiate between the rocks by means of chemistry and Coleman's diagrams alone are insufficient to determine whether or not a rock is a metamorphic peridotite. Coleman (1977) generalised that the average MgO/(MgO + FeO) ratio for dunite is 0.86, for harzburgite 0.85, and for lherzolite 0.84. The ratio for the metamorphic peridotite in the NEJXO is lower and ranges from 0.77 to 0.84. The difference of AI203 and the variation of other compositions within various metamorphic peridotite are very distinct. The REE compositions of the NEJXO are listed in
/
d
Averagemetamorphic 41- ~ h a r z b u r g i t e fZ_'e,^ I [ ~:)'1 Iv _}_ULtramafic 37 Averagemetnrnorphicduni'te 9 [ I I 20 40 60 I00 x FeO 1FeO Wt% + MgO
~5
~cumulate
I
BO
Fig. 6. Comparison of metamorphic peridotites and cumulates from NEJXO with Coleman's analogies. Nos are the same as in Table 2.
Table 2, and the REE patterns for NEJXO display some characters as follows. 1. All patterns of the metamorphic peridotite and cumulate ultramafic rocks show an inclined-to-right abundant type, suggesting they have a relation in the origin, but differing from the patterns of Coleman's metamorphic peridotite and resembling the REE pattern for the Proterozoic ophiolite from Yanbian, Sichuan Province (Li et al. 1983). Thus, we may consider this inclined-to-fight type may be distinct and differs from the younger ophiolites (Fig. 7). 2. ~:LREE, YHREE, YREE, (LREE/HREE)N, (La/Yb)N, (Ce/Yb)N, (La/Sm)N and 6Eu are listed in Table 3. All chrondrite normalised REE ratios for NEJXO are more than Coleman's homologues, and in LREE the even number REE is less than the next odd number REE but reversed in HREE. Coleman's curves are smoother than those for NEJXO. 3. The + 6 E u for metamorphic peridotite from NEJXO clearly differs from - ~ Eu for ultramafic cumulates from NEJXO but are similar to Coleman's metamorphic peridotite. 4. On the whole, the REE patterns of the members for NEJXO manifest a synchronism trend but can be divided into two groups: nos 1, 4 and 6 and nos 2, 3 and 5 (Fig. 7). The REE of the latter is more than that of the former and the curves of the latter are smoother than those of the former, but those of the cumulate ultramafic rocks lie between the two. The petrology, mineralogy, petrochemistry and REE content together indicate that the older mantle is more active than the younger mantle. The older earth crust is also thinner than the younger and the magma derived from the old mantle is close to the mantle in space and composition. The differentiation condition of the older is less than for the younger. In short, the older metamorphic peridotite and ophiolite are different from those which are younger. I wonder whether, for example, the
244
Zrlou GUOQING 17
J6~.'~
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\,\.~ ........ ~,.. ---..
•. • •
1
3
1
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..... , ......,..
~ •~
~
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e.""
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.
.
.
~.
..
.
.........
r~
0
*
\
1" X
/% "'~ -
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-----
1 2
"\./" ......... -
--
4 -
5
..........
Bo
I
I
La
Ce
J Pr
I
I
i
I
I
I
I
Nd
Pm
Sm
Eu
Gd
Tb
Dy
I ~ klo
Er
6
I
1
I
Tm
Yb
Lu
Fig. 7. Chrondrite normalized REE patterns for metamorphic peridotites and cumulates from NEJXO. 1. Col•man's metamorphic peridotite; 2. NEJXO metamorphic peridotites; 3. NEJXO ultramafic cumulates; 4. NEIXO marie cumulate; 5. NEJXO diorites; 6. NEJXO diabases. Nos are the same as in Table 2.
two types of kinkbands of olivines, the two-fold REE patterns, and the duality of petrochemistry of the metamorphic peridotite within the AFM and ACM diagrams for NEJXO, suggest there were two great activities or/and duality of the Proterozoic mantle. However, the data available for the Proterozoic mantle are so poor that I cannot speculate on this.
ANOTHER OPHIOLITE-- THE XIXAN OPHIOLITE Prof. Zhou Xinmin (personal communication) believes that the NEJXO is one ophiolite and the Xixan ophiolite is another. After comparing the two, I agree with him. The reasons for this area are: (a) as a boundary at Zhanggongshan to the north, the tectonic line at southern Anhui Province is east-west, at northeastern Jiangxi is N.E.-S.W.; (b) NEJXO is accompanied by a suite of epimetamorphic CA volcanic rocks, but the Xixan ophiolite is surrounded by several Xuefeng granites (about 1000 Ma), which indicate that their geological setting and developments are different; (c) their rock associations are also distinct. The pillow lava for Xixan ophiolite is well developed but pillow for NEJXO has not yet been found; (d) high temperature and high pressure metamorphic rocks have not yet been found in Xixan Ophiolite.
HIGH TEMPERATURE-HIGH PRESSURE METAMORPHIC ROCKS WITHIN NEJXO AND A DISCUSSION ON SUPERIMPOSITION METAMORPHISM Miyashiro's (1961) "paired metamorphism" theory has advanced the understanding of the geology and metamorphic petrology, especially catering for the collision mechanism on plate tectonics. Recently, besides some Japanese geologists, others, e.g. England and Richardson (1977), Nisbet and Fowler (1982) and Guo et al. (1980, 1983) have proposed different expositions on paired metamorphism. However, high temperature (I do not call it low pressure) high pressure metamorphic rocks are still being discovered one after another, even in the Proterozoic, for example, occurring within NEJXO. But we cannot reject every aspect of the paired metamorphism theory. The view of "cold, solid, allochthonous, and tectonic emplacement" has long been dominant in the study of ophiolite. Coleman (1984) said that "It seems likely that at least some of the compositional variations found in Tethyan Ophiolites could have resulted from magma changes during the collision and translation of newly-formed oceanic crust where water or xenoliths of altered continental crust could be easily introduced into magma chambers in the terminal stage of magmatic activity", which suggests that there is a possibility of
Features of northeastern Jiangxi ophiolite (NEJXO)
N
"autochthonous, magmatic, and hot emplacement" for a smaller ocean basin ophiolite against the setting of the continental margin. I believe that the high temperature metamorphic rocks in northeastern Jiangxi relate directly to the NEJXO. The reasons are as follows: (a) there are various hornstones as inclusions in the serpentinite of the NEJXO marbles, for example, garnet hornstone at Zhong Cun and Maoqiao, garnet-quartz hornstone, augite hornstone, andalusite--biotite hornstone, clinohumite(?) chrisotite hornstone and diopsidite at Maoqiao, tremolite hornstone at Xiwan, and phlogopite-melilite marble at Zhangshudun. The microprobe data of melilite and diopside are shown in Table 4. (b) There are no granites at Zhangshudun where melilite marble develops next to the gabbro. The hornstones at Xiwan, far away from a small granite body, are surrounded by the serpentinite. Those hornstones and diopsidites lie near the NEJXO at the northern side of a fault and are at a distance from the Damaoshan granite on the other side of the fault at Maoqiao. Furthermore, thermal metamorphism of many xenoliths of sedimentary rocks within the Damaoshan granite and of the country rocks surrounding the granite body are not as strong as the above• (c) The metamorphism of easily metamorphosed serpentinite and volcanic rocks is not caused by the granite. (d) These metamorphic rocks, either high-temperature or high-pressure, always accompany the NEJXO and, together with the NEJXO, are dismembered by tectonics but the granites are not affected. Thus, these metamorphic rocks directly relate to the NEJXO. The high pressure metamorphic rocks are aragonitejadeite-glaucophane schist at Xiwan and schistose lawsonite marble at Zhangshudun and other places.
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Table 4. Microprobe da t a and calculated formulae for diopsides at Ma oQ i a o and melilite from marble at Zha ngs hudun
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Diopsides Ma o 201 SiO2 TiO2 Al203 Cr203 FeO* MnO MgO CaO Na20 K20 Total Si Ti Al Iv Al vl Cr Fe3+ Fe 2+ Mn Mg Ca Na K Total Oxygens
55.20 0.04 0.29 -0.40 -19.62 24.18 0.18 -99.91 1.980 0.002 0.013 --. 0.013 -1.049 0.928 0.013 -3.998 6
Melilites Z R 248
52.25 0.40 2.84 0.10 2.93 0.06 16.41 24.50 0.18 0.08 99.75
36.85 0.06 38.49 0.05 0.33 0.06 0.03 22.90 0.04 -98.81
1.916 0.001 0.084 0.039 -.
. 0.090 0.002 0.896 0.963 0.013 0.004 4.018 6
1.544 0.003 -1.902 0.003 . 0.013 0.003 0.003 1.028 0.005 -4.504 7
37.68 -31.58 -0.86 0.02 0.05 24.88 0.01 -95.08
38.45 0.07 30.41 0.06 0.37 -0.03 23.32 0.04 0.005 92.78
1.653 --1.635 --
1.723 0.003 -1.605 0.003
0.032 0.007 0.032 1.171 --4.530 7
0.013 -0.003 1.120 0.003 -4.473 7
.
246
ZHOU GUOQING Table 5. Analysis and calculated formula of crossite from aragonitc-jadeite glaucophane schist at Xiwan
SiO2 TiO2 AI203 Cr203 Fe~O3 FeO* NiO MnO MgO CaO Na20 K20 LOI Total
Xiwan Crossite
Xi Winchite
57,46 0.10 6.41 . 4.90 3.90 . 0.17 14.09 3.61 6.27 0.28 2.14 99.33
56.49 0.14 1.17 .
12-2 Magnesioarfvedsonite
Xi-12 Cummingtonite
56.67 0.06 0.56 .
54.50 0.01 0.75
. . I 1.50
.
.
. 16.83
12.22 . 0.45 17.68 2.28 5.65
.
JX Crossite
12 Glaueophane
50.83 0.11 3.40 0.05 . 7.24 0.04 0.13 24.02 10.28 2.76 . . 98.86
60, I 7 -2.70
59.38 -5.20
-
-
-
-
. 7.77 --20.92 1.29 7.14
12.96 --14.87 2.24 5.35
95.57
0.70 21.27 0.83 1.00 . . 95.89
99.99
100.00
Si Ti A1w A1vl
7.95 0.01 0.04 1.01
7.99 0.02 -0.19
8.19 0.01 -0.09
7.91 -0,09 0.03
7.15 0.01 0.56 --
7.88 -0.12 0.29
7.69 -0.31 0.48
Fe 3+ Fe 2+
0.51 0.45
-1.36
-1.48
-2.04
0.02 0.84
-0.85
-1.40
0.02 2.90 0.53 1.68 0.05 15.15 24
0.07 4.09 0.26 1.85 . 15.83 23
0.05 3.81 0.36 1.58
0.08 4.61 0.13 0.28 . 15.17 23
0.03 5.03 1.55 0.75
-4.08 0.18 1.81
-2.87 0.31 1.34
15.21 23
14.40 23
Mn Mg Ca Na K Total Oxygens
0.54 19.40 1.70 6.79 . . 97.73
.
J2x- 12 Szechenite
. .
. .
.
. 15.56 23
. .
.
.
15.96 23
cut the melilite. Therefore it is possible that they may occur in the Proterozoic era as the NEJXO, but the high-temperature metamorphism is earlier than the highpressure metamorphism, which I termed as "superimposition metamorphism" or "Remetamorphism". I think that Miyashiro's "paired metamorphism" theory only suits the collision mechanism of the convergent plates at the adjacent space in the same age, i.e. the high-T and high-P of paired metamorphism occur at the continent side of the collision zone and the ocean side, respectively, during the period when the collision is
The compositions of the crossite and other amphiboles, lawsonite, jadeite and aragonite are shown in Tables 5 and 6, respectively. It is obvious that the compositions of these minerals vary on a larger scale and are largely controlled by the original rocks and the CaO of the lawsonite in the marble, SiO2 in melilite and jadeite are slightly more than the common values. It is interesting to note that the crossite may come from the earlier tremolite in the hornstone, the schistose lawsonite marble occurs along a compression fault within the melilite marble and the lawsonite
Table 6. Microprobe data and calculated formulae of aragonite, jadeite and lawsonite Lawsonite
Jadeite Jx-05: X-12
Z R Z 203-5 SiO2 TiO2 A1203 Cr~O3 FeO* MgO CaO MnO NiO Na20 K20 Total Si Ti AI Fe 2+ Mg Ca Mn Na K Total Oxygens
40,58 0,02 33.78 -0.24 0.19 25.07 0.06 --0.02 99.96 1.928 0.001 1.891 0.009 0.013 1.275 0.002 -0.001 5.120 8
40.35 0.03 34.01 -0.18 0.05 25.28 0.05 -0.03 0.01 99.99 1.918 0.001 1.906 0.007 0.003 1,288 0,002 0.003 0.001 5.129 8
40.54 . 31.69 -0.17 2.71 24.57 . --0.33 100.01
69.20 .
. 22.19 0.03 0.06 0.02 0.28
.
. 0.01 7.89 0.30 99.98
1.933 2.233 . . . 1.781 0.844 0.006 0.002 0.192 0.001 1.254 0.010 . . . -0.494 0.020 0.012 5.186 3.597 8 6
68.52 . 26.83 0.02 0.02 0.03 0.54 . -4.01 0.01 99.98 2.175 . 1,004 0.001 0.001 0.018 . 0.247 -3.446 6
Aragonite Jx-05 Xi12-2 67.67 . 17.64 -0.05 0.05 0.17 -7.49 -93.07 2.330 0.717 -0.002 0.006 0.501 -3.556 6
0.06 0.04 0.07 0,17 0,03 52.42 0.01 0.04 0.16 -53.00
0.06 0.04 0.02
0.15 0.19 0.04
0.I0 0.01 55.50 0.03
0.13 0.02 54.45 0.12
0.06
0.05
55.82
55.15
Features of northeastern Jiangxi ophiolite (NEJXO) advancing, but the superimposition metamorphism is more suitable for the "open(tension)-closure (compression) mechanism" of the smaller ocean basins located at continental margins, that is, "superimposition metamorphism" means that the high-temperature metamorphism resulting from magmatism during the "open" (early) stage is superimposed on the highpressure metamorphism resulting from tectonism during the "closure" (late) stage at the same place but at different ages. Both the superimposition metamorphism and paired metamorphism may together be termed as "bimetamorphism". CONCLUSIONS Here, the author has described and discussed in detail both the Proterozoic metamorphic peridotite and superimposition metamorphism. Although a lot of further work is required, we can say that older metamorphic peridotites must be different from those of younger ages in some aspects. Paired metamorphism only affects one side of the plate and can better explain metamorphism due to plate collision. Superimposition metamorphism suggested here appears to be able to explain metamorphism of small oceanic basins located at continental margins. REFERENCES Bureau of Geology and Mineral Resources of Jiangxi Province. 1984. Regional Geological Analyses of Jiangxi Province. Geological Memoirs, Ministry of Geology and Mineral Resource, People's
247
Republic of China. Regional Geology 2nd. Geological Publ. House, Beijing. Coleman, R. G. 1977. Ophiolite: Ancient Oceanic Lithospere? Springer, Berlin. Coleman, R. G. 1984. The diversity of ophiolites. Geol. Mijnbouw 63, 141-150. England, P. C. and Richardson, S. W. 1977. The influence of erosion upon the mineral facies of rocks from different metamorphic environments. J. Geol. Soc., Lond. 134, 201-213. Gass, I. G. 1982. Ophiolite. Scient. Amer. 247. Guo, L. Z., Shi, Y. S. and Ms, R. S. 1980. The geotectonic framework and crustal evolution of South China. In: Scientific Papers on Geology for International Exchange, Prepared for the 26th International Geological Congress. Tectonic Geology and Geological Mechanics. Geological Publ. House, Beijing. Guo, L. Z., Shi, Y. S. and Ma, R. S. 1983. On the formation and evolution of the Mesozoic-Cenozoic active continental margin and island arc tectonics of the western Pacific Ocean. Acta Geol. Sin. 1, ll-21. Huang, T. K., Chang, C. K., Chang, C. M. and Chen, K. M. 1965. On Eugeosynclines and Miogeosynclines of China and their Polycyclic Development. Collected works of geological sciences, C series. Ministry of Geology, People's Republic of China. Li, J. L., Zhang, F. Q. and Wang, S. X. 1983. REE distribution patterns of some rocks in the Proterozoic ophiolite in Yanbian, Sichuan. Petrol. Res. 3, 35-44. Miyashiro A. 1961. Evolution of metamorphic belts. J. Petrol. 2, 22-311. Nicolas, A. and Jackson, E. D. 1972. Repartition en deux provinces des peridotites des chains alpines longeant la M6diterranee: implications geotectoniques. Bull. Soc. Min. Petr. 52, 479-495. Nisbet, E. G. and Fowler, C. M. R. 1982. The thermal background to metamorphism sample two dimensional conductive models. Geosci. Can. 9, 161-164, 208-214. Ren, J. S., Jiang, C. F., Zhang, Z. K. and Qin, D. Y. 1980. The tectonic evolution of China. Scientific Papers on Geology for International Exchange, Prepared for the 26th International Geological Congress. Tectonic Geology and Geological Mechanics. Geological Publ. House, Beijing. Shu, L. S. and Li, Y. J. 1987. On terrane tectonics of North JiangXi. Geol. JiangXi 1, 31-37. Zhu, X., Huang, C. K., Rui, Z. Y., Zhou, Y. H., Zhu, X. J., Hu, C. S. and Mei, Z. K. 1983. Porphyry Copper Deposits at Dexin Jiangxi. Geological Publ. House, Beijing.
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The discovery and significance of the northeastern Jiangxi Province ophiolite (NEJXO), its metamorphic peridotite and associated high temperature-high pressure metamorphic rocks ZHOU GUOQING Department of Earth Sciences, Nanjing University, Nanjing, People's Republic of China Abstract--The NEJXO with a N.E.-S.W. elongation occurs in the mid-Lower Qigong Group, under which lies the Jiuling Group (1401 Ma) and above which lies the Shangshu Group (817 + 87 Ma), so that the age of NEJXO is defined to be Proterozoic between 1401 Ma and 817 + 87Ma. The sediments of the Jiuling Group show evidence of continental derivation, but the Qigong Group and Shangshu Group are characterised by CA volcanic rocks and probably represent a gradually growing island-arc. Thus, we regard the NEJXO as occurring in a back-island-arc basin between the ancient continent and the island-arc. On the whole, the main members of dismembered ophiolite are all present. The metamorphic periodotite present in them, is considered to be especially important, because it may be the sole representative of the older mantle present and it differs from those younger. The high-T metamorphic rocks associated with the NEJXO are various hornstones and melilite marble, whereas the high-P metamorphic rocks are aragonite-jadeite--glaucophane schist and schistose lawsonite marble. From the fact that high-P metamorphism was superimposed on the high-T metamorphic rocks, it may be suggested that early tension (at opening stage) and late compression (at closing stage) occurred during the development of the basin.
INTRODUCTION Tim NORTHEASTERNregion of Jiangxi Province adjacent to Anhui and Zhejiang Provinces, was known as the northeastern margin of the "Jiangnan earth axis" (Huang et al. 1965), the "Jiangnan Dongan-Xuefeng island-arc folded system" (Guo et al. 1980) and the "Yangzhi paraplatform" (Ren et al. 1980). In this area, more than 100 basic and ultrabasic bodies outcrop from Yiyang, Dexing, Wuyuan in Jiangxi to Xixan in Anhui in a S.W.-N.E. direction. Zhu et al. (1983) suggested that these bodies formed at the Yanshan Stage and marked the "N.E. Jiangxi Deep Fault Zone". At the First Chinese Basic, Ultrabasic Rocks and Ophiolite Symposium, 1981, I first proposed that those basic and ultrabasic bodies occurring in N.E. Jiangxi together with the Proterozoic meta-volcanic rocks represent a regular ophiolite suite and named this the "northeastern Jiangxi ophiolite" (NEJXO) (Fig. 1).
THE PROOF OF NEJXO At the Penrose Meeting, 1972, an ophiolite suite was defined as an association of a specific rock series representing development of the oceanic crust (Coleman 1977, Gass 1982). The evidence for interpreting the occurrence of an ophiolite suite in N.E. Jiangxi is as follows: 1. As a whole, the main members of an ophiolite suite are present, such as metamorphic peridotite, cumulates, diabase and metalavas (as well as silicalites), etc., but in places one or two of them may be absent as a result of dismembering; 2. The presence of fragments of members mentioned above can be found in some metabreccias and volcanic
breccias, which indicates that before some metamorphic event during the Proterozoic, there was already a close relation between those members; 3. The members always occur together along the N.E.-S.W. belt in space; 4. Among the members, there is an intimate relation in the composition and origin, which will be demonstrated in the following sections.
THE OCCURRENCE AND AGE OF NEJXO Based on the following reasons, I believe that the NEJXO developed in the mid-late Proterozoic: 1. The NEJXO was cut through the middle by the Damaoshan Granite which has been determined to be 135 Ma in age (K-Ar, biotite, 1974); also, it intruded into the Triassic-Jurassic Anyuan Group and an ultrabasic body. Therefore the NEJXO is older than the Yanshan Period (about 100 Ma); 2. We cannot see any contact between the NEJXO and the Sinian, Cambrian or Ordovician strata, but there is a tectonic contact between the NEJXO and the Permian, Triassic as well as Jurassic. However, the NEJXO always occurs within the Proterozoic epirocks and fragments of its members are associated together in metabreccias and metavolcanic breccias; 3. The schistosity that developed within phyllite passed through the serpentinite; 4. The NEJXO lies in the mid-Lower Qigong Group which overlies the Jiuling Group and underlies the Sanshu Group. The Sanshu contains gravels of the serpentinite in the basal conglomerate. According to the Bureau of Geology and Mineral Resources of Jiangxi
237
238
ZHOU GUOQrNG the NEJXO must have occurred in a setting related to an island-arc.
I
PETROLOGY AND MINERALOGY OF NEJXO
Is there any difference between Proterozoic and postProterozoic ophiolites? To this question the NEJXO may be able to provide some answers. I believe that the Proterozoic metamorphic peridotite is especially worthy of further attention.
1. Metamorphic peridotites
I I I JlK/'"L'~"uu"uuu"
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/
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Fig. I. Geological sketch map of the northeastern Jiangxi ophiolite suite. 1. Jiuling Group; 2. Qigong Group; 3. Sinian~)rdovician system; 4. Mesozoic volcanic rocks; 5. Mesozoic basin; 6. Yanshanian granites; 7. Xuefengian granites; 8. Ultrabasic bodies representing NEJXO, plus high-temperature metamorphic rocks (T) and high-pressure metamorphic rocks (P); 9. Main faults.
Province (1984) and Shu and Li (1987), a Rb-Sr isochron age of tuffaceous phyllite in the lower Shuangqiac~han Group (corresponding to the Jiuling Group) is 1401 Ma old, and a Rb--Sr age for metarhyolite in the top of the Shanshu Group is 817 _+ 87 Ma old. Xu Bei (personal communication), obtains a Sm-Nd isochron age of 915 _ 34 Ma for the NEJXO. From the above it is clear that the age of the NEJXO is about 900-1000 Ma. Care must be taken when applying petrochemistry and geochemistry to distinguish the tectonic setting of a ophiolite, and a detailed geological analysis is required, especially for old, dismembered and metamorphosed ophiolite. Recently, Shu and Li (1987) proposed that the NEJXO melange marks a terrane collage zone between the Jiuling Terrane and the Huaiyu Terrane and the zone coincides with Zhu's (1983) N.E. Jiangxi Deep Fault Zone (Fig. 1). The Jiuling Terrane is mainly made up of the Jiuling Group, which contains epimetamorphic sandstone, siltstone, pelite and meta intermediate-basic volcanic rocks at the top. These sediments come from the north, and are of continental provenance. The Huaiyu Terrane consists of the Qigong Group and the Sangshu Group. The Qigong Group contains spilite-keratophyre series, metabasalt-andesite series and abundant pyroclastics, and the Shangshu Group mainly contains a calc-alkaline series made of basalt, andesite and rhyolite. Both these groups appear to represent a gradually matured island-arc, therefore
Metamorphic peridotites occur at Xiwan, Dazishan and Maoqiao, etc., and consist mainly of harzburgite, dunite (Xiwan, Dazishan), olivine-websterite and augite-peridotite (Maoqiao). They are fresher than the cumulate ultramafic rocks which have been serpentinised. In Xiwan, abundant inclusions of harzburgite are included in the serpentinised cumulate ultramafic rocks which is one of the obvious differences. The foliations of the harzburgite inclusions with hobnail strike at 90°-270 °, 45°-225 ° and 30°-210 °, but the schistosity of the serpentinised cumulate ultramafic rocks strike along 180°-360 °. The olivines of the dunite at Maoqiao appear with kink-bands. The refractive indices of the olivine measured by the immersion method are Np = 1.6520, Nm = 1.6659, Ng = 1.6827, Ng-Np = 0.0293, (+)2V = 84.5, Fo = 93 __+1 (sample R80). The electron microprobe data and calculated formulae of the olivine (Fo: 89.17) from sample Mao 191 are listed in Table 1. Table 1. Microprobe analyses and calculated formulae for olivine, enstatite and chrome spinel in dunite (Mao 191)
SiO2 TiO2 A1203 Cr203 NiO MgO FeO* MnO CaO Na20 K20 Total
Olivine
Enstatite
Chrome spinel
38.25 0.04 0.20 0.09 0.44 50.33 10.53 0.11 -0.07 -100.06
54.07 0.04 2.00 0.13 0.08 35.92 7.32 0.15 0.16 0.17 -100.04
0.22 0.39 23.32 39.51 0.14 12.12 23.98 0.23 0.03 0.09 -100.03
Mineral formulae Si 0.935 Ti 0.001 AI -Cr 0.001 Fe ~+ -Fe 2+ 0.215 Ni 0.009 Mg 1.835 Mn 0.002 Ca -Na 0.002 K
Total Oxygens
- -
3.000 4
1.889 0.001 0.004 0.002 -0.214 0.002 1.871 0.004 0.006 0.006 - -
4.000 6
0.001 0.009 0.884 0.959 0.276 0.433 0.004 0.555 0.006 0.001 0.006 - -
3.134 4
Features of northeastern Jiangxi ophiolite (NEJXO) Along fractures in the dunite, olivines have been transformed into clusters of enstatite (Fig. 2). The microprobe data and calculated formulae of the enstatite from Mao 191 are also listed in Table 1. After the formation of the enstatite, some diopside, phlogopite and carbonates are formed in succession. The last mineral to form is serpentine. From the above evidence it can be suggested that after the dunite was fractured, it had experienced "dry" heating followed by "wet" chemical alteration. Considering only heating and neglecting others, the transformation of olivine to enstatite should occur at 1557°C, if considering pressure and other factors as well this transformation temperature must be higher. The composition and calculation of chrome spinel in Mao 191 can be seen in Table 1. A special and rare characteristic "grated" texture (Fig. 3) present in the dunite is very interesting and differs from that shown by meteorites. It consists nearly all of olivine bars probably caused by cross-gliding. The harzburgite occurring at Xiwan has a protogranular texture, the kink bands of the olivines are clear and common, and sometimes display fan-shaped bands (Fig. 4). In this case, the boundaries between the kink bands are not as obvious as those in younger harzburgites and it appears they had undergone another phase of plastic deformation. This is another important feature of the Proterozoic metamorphic peridotite. In the enstatite, kink bands, undulatory extinction, glide-twins and exsolution lamellae can be seen. The ovoid olivine or diopside may be encircled by enstatites, which suggests that magmatism had occurred before the deformation of the metamorphic peridotite. The lherzolite-olivine websterite is fresh, coarsegrained and is made up of olivine, enstatite, diopside and chrome spinel. Since the percentage of minerals in them is variable, the rocks form a series from lherzolite to olivine-websterite. Kink bands, exsolution lamellae, two directional glides and glide twins in both clinopyroxenes and orthopyroxenes can be seen. As the exolution lamellae are kinked, it is suggested that the exsolution had been formed before kinking. Some enstatite exhibits an irregularly clear rim, but some olivines are grain recrystallized from fine olivines crushed from some earlier coarse olivines. Coleman (1977) defined a metamorphic peridotite as a peridotite with tectonic fabric, mainly harzburgite, representing a mantle material that has undergone a subsolidus rheomorphism rather than chemistry. The metamorphic peridotite without cumulate fabric in the area fits into this definition. 2. Cumulative rocks
Cumulate rocks present include augite-peridotite, chromitite, gabbro-diorite and plagioclasite. The rocks commonly show intense alteration, in which the augite peridotite have nearly all been transformed into serpentine. Among the cumulate rocks, the augite--peridotite is dominant, others are subordinate. All these rocks
239
exhibit cumulate texture but bandstructure is not developed. These features suggest that the crust was thinner at that time and the magma was derived from the mantle close by. This magma was more ultrabasic and more active. Diabase is often present as in tectonic blocks mixed with serpentinite and sometimes occur as individual dykes intruding into phyllite. Sheeted dyke swarms have not been seen. 3. Lavas
The lavas are represented by the spilite-keratophyre series and the basalt-andesite series. The spilitekeratophyre rocks have been altered into "green stone series". It is significant that we have not yet found any pillow lava. However, volcanic pyroclastic rocks accompanying the lavas are highly developed. Other members within the NEJXO are the tuffaceous phyllite, zoned silicalites, siliceous shales, red jasper, calc-shales and limestone.
THE PETROCHEMISTRY AND GEOCHEMISTRY OF NEJXO
Bulk rock chemical analyses and analysis of the crossite were done by the Analysis Center of the Department of Earth Sciences, Nanjing University. Chemical analyses of other minerals were carried out on a JEOL JXA-733 Electron Microprobe at this Maanshan Mining Research Institute, Ministry of Metallurgical Industry, and a Hitachi X650 Scanning Electron Microanalyzer with EDAX 9100 in the Center of Material Analysis, Nanjing University. Bulk rock REE analyses were made by a GP3.5-E Inductively Coupled Plasma Quantometer in the Yichang Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences. We should note that the mantle is not invariant and that the ophiolitic metamorphic peridotites, one of mantle material, and other related ophiolites are also not invariant. Thus, the petrochemistry and geochemistry of ophiolites of various ages must have in addition to their general characters, inherited characters and individuality and variability. This information can be obtained by comparing the Proterozoic ophiolite with those from the Meso-Cenozoic era, but this comparison ought not to be mechanical. On the other hand, the Proterozoic metamorphic peridotite may more approach the protomantle and is therefore an important source for information regarding the old mantle to which we must pay great attention. We shall emphasise the petrochemistry and geochemistry of the metamorphic peridotite within NEJXO. A comparison of the metamorphic peridotites of Proterozoic ophiolite with those occurring mainly in younger ophiolites, can be made by using Coleman's (1977) AFM, ACM, and SiO2-100 x FeO*/(FeO + MgO) diagrams, and similarly comparison between the metamorphic peridotites and cumulates can be made. Results
2
3
4
0.80
0.802 2.768 0.464 1.715 0.483 0.207 0.469 0.085 0.462 0.078 0.227 0.038 0.202 0.016 1.417
0.82
0.409 0.439 0.166 0.214 0.095 0.061 0.072 0.035 0.062 0.020 0.042 0.083 0.018 0.007 0.173
0.469 1.058 0.381 1.543 0.405 0.216 0.477 0.081 0.436 0.083 0.245 0.036 0.237 0.018 1.339
0.78
1.109 1.547 0.372 0.943 0.198 0.044 0.130 0.044 0.122 0.031 0.081 0.025 0.050 0.010 0.528
0.83
5
6
7
8
0.836 2.605 0.306 0.660 0.172 0.066 0.095 0.037 0.085 0.020 0.050 0.018 0.023 0.059 0.276
0.77
0.853 2.117 0.348 0.857 0.285 0.155 0.364 0.085 0.541 0.105 0.350 0.057 0.474 0.036 1.969
0.87 1.279 3.826 0.488 1.458 0.492 0.173 0.807 0.162 i.046 0.218 0.761 0.096 0.492 0.064 4.409
0.80
0.84
39.66 49.08 48.52 40.85 0.151 0.057 0.302 0.220 3.49 0.90 4.91 1.57 5.88 2.28 1.64 6.63 4.58 2.56 2.85 0.87 0.139 0.155 0.124 0.050 32.63 3 1 . 3 1 1 7 . 6 3 37.16 3.40 5.52 20.95 -0.25 0.04 0.24 -0.04 0.005 <0.005 0.08 0.031 0.017 0 . 0 3 1 0.960 9.44 8.34 2.04 12.37 99.69 100.26 99.24 100.76
9
!.109 1.303 0.414 0.815 0.405 0.060 0.217 0.102 0.148 0.057 0.131 0.057 0.114 0.013 1.260
0.81
33.88 0.057 1.16 3.70 4.47 0.098 33.38 0.40 0.16 0.07 0.017 21.62 99.01
10
0.938 0.977 0.563 1.286 0.345 0.081 0.373 0.119 0.305 0.081 0.184 0.057 0.088 0.018 1.496
0.78
39.36 0.069 1.68 7.69 3.42 0.072 35.74 0.03 0.02 0.005 0.017 12.44 100.54
11
0.85
40.78 0.080 0.82 6.98 0.50 0.060 37.55 0.38 0.08 --12.54 99.77
12
0.85
38.88 0.030 0.72 6.16 1.23 0.110 38.16 0.25 --0.01 13.96 99.51
13
14
1.364 1.628 1.076 1.543 0.655 0.147 0.746 0.179 1.655 0.148 0.577 0.105 0.667 0.026 5.591
0.72
0.49
51.84 52.30 0.245 0.950 15.37 18.38 1.09 1.78 3.03 4.57 0.124 0.12 10.25 5.99 8.70 9.66 4.40 2.89 0.16 0.45 0 . 0 3 1 0.07 4.22 3.00 99.46 100.16
15
16
17
18
19
20
4.434 6.838 1.986 6.602 1.380 0.466 1.822 0.298 2.004 0.367 1.312 0.158 0.834 0.132 11.811
0.46
0.38
3.837 4.775 7.327 5.780 2.400 2.152 6.516 8.403 1.639 2.070 0.544 0.734 1.909 2.776 0.391 0.434 2.701 2.701 0.455 0.620 1.662 2.011 0.228 0.245 1.405 !.317 0.185 0.202 13.386 18.898
0.49
0.49
0.26
0.29
53.60 52.60 51.30 53.03 46.21 51.08 0.866 0.797 1.220 1.010 1.980 0.650 16.15 15.89 14.60 14.36 13.29 13.44 1.77 2.13 3.95 1.35 5.27 3.54 5.34 5.26 6.70 6.90 10.48 4.36 0.139 0.129 0.201 0 . 2 1 0.23 0.42 5.95 6.90 6.50 7.82 6.12 10.64 8.80 9.75 7.85 10.20 9.20 9.05 3.46 2.84 2.86 2.62 2.49 2.50 0.80 0.58 0.29 0.28 0.13 0.58 0.129 0.112 0.129 0.17 0.28 0.08 2.12 2.30 3.48 1.98 4.12 3.06 99.12 99.29 99.08 99.93 99.80 99.40
21
0.59
52.83 0.990 14.63 1.96 6.12 0.15 7.77 9.70 2.89 0.29 0.23 2.34 99.90
22
23
0.50
0.27
55.16 41.81 0.580 4.290 17.45 14.30 2.10 5.05 5.95 13.48 0.27 0.19 5.12 6.62 5.11 9.31 2.92 1.21 1.06 0.19 0.17 1.08 3.94 3.22 99.83 100.75
1-4, metamorphic dunites; 5-6, augite peridotites; 7, olivine websterite; 8, harzburgite; 9-12, cumulate augite peridotites; 13-14, cumulate gabbros; 15-16, diorites; 17-18, diabases; 19-21, meta-basalts; 22, meta-andesite; 23, ultrabasic lava.
FeO+MgO La Ce Pr bid Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y
MgO
CaO Na20 K20 P205 LOI Total
MgO
TiO2 A1203 Fe203 FeO MnO
$iOz
1
39.14 45.12 44.96 40.34 0.044 0.076 0.126 0.025 0.90 1.03 i.42 0.64 6.27 4.07 5.35 5.49 2.60 4.11 3.96 2.27 0.098 0.139 0.160 0.217 37.44 31.78 30.55 34.89 0.I0 4.15 4.05 2.33 <0.01 0.11 0.07 0.02 <0.005 0.01 0.01 0.005 0.031 0 . 0 3 1 0.031 0.017 13.10 8.84 8.80 12.66 99.74 99.47 99.77 98.90
Table 2. Chemical analyses and REE composition (ppm) for NEJXO
N
bo 4~
Fig. 2. The clusters of the enstatites transformed from olivines in dunite from Maoqiao. Crossed polars, 1 bar = 0.2 mm. En, Enstatite; Tr, Tremolite.
Fig. 3. The "grated texture" composed of crossed olivine bars in dunite from Maoqiao. Crossed polars, 1 bar = 0.2 mm. O1, olivine.
Fig. 4. The fan-shaped kink bands of the olivines in the harzburgite from Xiwan. Crossed polars, 1 bar = 0.2 mm. O1, olivine.
241
Fig. 8. The melilite marble from Zhangshudun. Crossed polars, 1 bar = 0.2 mm. Mel, melilite: Cc, calcite
Fig. 9. The glaucophane schist from Xiwan. Crossed polars, 1 bar = 0.2 ram. GI, glaucophane (crossite).
242
Features of northeastern Jiangxi ophiolite (NEJXO) At
/ CaO
0
243
FeO
K°maTe'7 ' .(!i!.!t!!. i i ~ i ~L i e t' ~ ' ' Na20* Kz0
MgO
Fig. 5. (a) AFM diagram comparing the metamorphic peridotite, maflc and ultramafic rocks from NEJXO with Coleman's metamorphic peridotites, marie and ultramaric cumulates (1977). (b) Triangular diagram of MgO-CaO-A1203 comparing NEJXO containing metamorphic peridotite with Coleman's (I 977), Nicolas' and Jackson's (1972) metamorphic peridotites. (a) The distribution range of dunite and harzburgite, (b) the distribution of iherzolite. • NEJXO; the numbers are the same as in Table 2.
are given in Table 2, and plotted in Figs 5 and 6. It can be seen that both the metamorphic peridotite and ultramafic cumulates plot in Coleman's "metamorphic peridotite area" and "cumulate area", or even outside these two areas (e.g. no. 7). It appears rather unsuitable to differentiate between the rocks by means of chemistry and Coleman's diagrams alone are insufficient to determine whether or not a rock is a metamorphic peridotite. Coleman (1977) generalised that the average MgO/(MgO + FeO) ratio for dunite is 0.86, for harzburgite 0.85, and for lherzolite 0.84. The ratio for the metamorphic peridotite in the NEJXO is lower and ranges from 0.77 to 0.84. The difference of AI203 and the variation of other compositions within various metamorphic peridotite are very distinct. The REE compositions of the NEJXO are listed in
/
d
Averagemetamorphic 41- ~ h a r z b u r g i t e fZ_'e,^ I [ ~:)'1 Iv _}_ULtramafic 37 Averagemetnrnorphicduni'te 9 [ I I 20 40 60 I00 x FeO 1FeO Wt% + MgO
~5
~cumulate
I
BO
Fig. 6. Comparison of metamorphic peridotites and cumulates from NEJXO with Coleman's analogies. Nos are the same as in Table 2.
Table 2, and the REE patterns for NEJXO display some characters as follows. 1. All patterns of the metamorphic peridotite and cumulate ultramafic rocks show an inclined-to-right abundant type, suggesting they have a relation in the origin, but differing from the patterns of Coleman's metamorphic peridotite and resembling the REE pattern for the Proterozoic ophiolite from Yanbian, Sichuan Province (Li et al. 1983). Thus, we may consider this inclined-to-fight type may be distinct and differs from the younger ophiolites (Fig. 7). 2. ~:LREE, YHREE, YREE, (LREE/HREE)N, (La/Yb)N, (Ce/Yb)N, (La/Sm)N and 6Eu are listed in Table 3. All chrondrite normalised REE ratios for NEJXO are more than Coleman's homologues, and in LREE the even number REE is less than the next odd number REE but reversed in HREE. Coleman's curves are smoother than those for NEJXO. 3. The + 6 E u for metamorphic peridotite from NEJXO clearly differs from - ~ Eu for ultramafic cumulates from NEJXO but are similar to Coleman's metamorphic peridotite. 4. On the whole, the REE patterns of the members for NEJXO manifest a synchronism trend but can be divided into two groups: nos 1, 4 and 6 and nos 2, 3 and 5 (Fig. 7). The REE of the latter is more than that of the former and the curves of the latter are smoother than those of the former, but those of the cumulate ultramafic rocks lie between the two. The petrology, mineralogy, petrochemistry and REE content together indicate that the older mantle is more active than the younger mantle. The older earth crust is also thinner than the younger and the magma derived from the old mantle is close to the mantle in space and composition. The differentiation condition of the older is less than for the younger. In short, the older metamorphic peridotite and ophiolite are different from those which are younger. I wonder whether, for example, the
244
Zrlou GUOQING 17
J6~.'~
/~\',,~.
//
~/"
\,\.~ ........ ~,.. ---..
•. • •
1
3
1
/
•
..o
..... , ......,..
~ •~
~
.....
\
. .....
e.""
"'..
' .. .
.
.
.
~.
..
.
.........
r~
0
*
\
1" X
/% "'~ -
/"
...
/.
-----
1 2
"\./" ......... -
--
4 -
5
..........
Bo
I
I
La
Ce
J Pr
I
I
i
I
I
I
I
Nd
Pm
Sm
Eu
Gd
Tb
Dy
I ~ klo
Er
6
I
1
I
Tm
Yb
Lu
Fig. 7. Chrondrite normalized REE patterns for metamorphic peridotites and cumulates from NEJXO. 1. Col•man's metamorphic peridotite; 2. NEJXO metamorphic peridotites; 3. NEJXO ultramafic cumulates; 4. NEIXO marie cumulate; 5. NEJXO diorites; 6. NEJXO diabases. Nos are the same as in Table 2.
two types of kinkbands of olivines, the two-fold REE patterns, and the duality of petrochemistry of the metamorphic peridotite within the AFM and ACM diagrams for NEJXO, suggest there were two great activities or/and duality of the Proterozoic mantle. However, the data available for the Proterozoic mantle are so poor that I cannot speculate on this.
ANOTHER OPHIOLITE-- THE XIXAN OPHIOLITE Prof. Zhou Xinmin (personal communication) believes that the NEJXO is one ophiolite and the Xixan ophiolite is another. After comparing the two, I agree with him. The reasons for this area are: (a) as a boundary at Zhanggongshan to the north, the tectonic line at southern Anhui Province is east-west, at northeastern Jiangxi is N.E.-S.W.; (b) NEJXO is accompanied by a suite of epimetamorphic CA volcanic rocks, but the Xixan ophiolite is surrounded by several Xuefeng granites (about 1000 Ma), which indicate that their geological setting and developments are different; (c) their rock associations are also distinct. The pillow lava for Xixan ophiolite is well developed but pillow for NEJXO has not yet been found; (d) high temperature and high pressure metamorphic rocks have not yet been found in Xixan Ophiolite.
HIGH TEMPERATURE-HIGH PRESSURE METAMORPHIC ROCKS WITHIN NEJXO AND A DISCUSSION ON SUPERIMPOSITION METAMORPHISM Miyashiro's (1961) "paired metamorphism" theory has advanced the understanding of the geology and metamorphic petrology, especially catering for the collision mechanism on plate tectonics. Recently, besides some Japanese geologists, others, e.g. England and Richardson (1977), Nisbet and Fowler (1982) and Guo et al. (1980, 1983) have proposed different expositions on paired metamorphism. However, high temperature (I do not call it low pressure) high pressure metamorphic rocks are still being discovered one after another, even in the Proterozoic, for example, occurring within NEJXO. But we cannot reject every aspect of the paired metamorphism theory. The view of "cold, solid, allochthonous, and tectonic emplacement" has long been dominant in the study of ophiolite. Coleman (1984) said that "It seems likely that at least some of the compositional variations found in Tethyan Ophiolites could have resulted from magma changes during the collision and translation of newly-formed oceanic crust where water or xenoliths of altered continental crust could be easily introduced into magma chambers in the terminal stage of magmatic activity", which suggests that there is a possibility of
Features of northeastern Jiangxi ophiolite (NEJXO)
N
"autochthonous, magmatic, and hot emplacement" for a smaller ocean basin ophiolite against the setting of the continental margin. I believe that the high temperature metamorphic rocks in northeastern Jiangxi relate directly to the NEJXO. The reasons are as follows: (a) there are various hornstones as inclusions in the serpentinite of the NEJXO marbles, for example, garnet hornstone at Zhong Cun and Maoqiao, garnet-quartz hornstone, augite hornstone, andalusite--biotite hornstone, clinohumite(?) chrisotite hornstone and diopsidite at Maoqiao, tremolite hornstone at Xiwan, and phlogopite-melilite marble at Zhangshudun. The microprobe data of melilite and diopside are shown in Table 4. (b) There are no granites at Zhangshudun where melilite marble develops next to the gabbro. The hornstones at Xiwan, far away from a small granite body, are surrounded by the serpentinite. Those hornstones and diopsidites lie near the NEJXO at the northern side of a fault and are at a distance from the Damaoshan granite on the other side of the fault at Maoqiao. Furthermore, thermal metamorphism of many xenoliths of sedimentary rocks within the Damaoshan granite and of the country rocks surrounding the granite body are not as strong as the above• (c) The metamorphism of easily metamorphosed serpentinite and volcanic rocks is not caused by the granite. (d) These metamorphic rocks, either high-temperature or high-pressure, always accompany the NEJXO and, together with the NEJXO, are dismembered by tectonics but the granites are not affected. Thus, these metamorphic rocks directly relate to the NEJXO. The high pressure metamorphic rocks are aragonitejadeite-glaucophane schist at Xiwan and schistose lawsonite marble at Zhangshudun and other places.
I
¢'4
N
I
~.~I~ Z
8
= e,i
~
o
I
oO
m
--
•
I
•
~: c4 ,o
--
--
245
+
o .q.
Table 4. Microprobe da t a and calculated formulae for diopsides at Ma oQ i a o and melilite from marble at Zha ngs hudun
I
N
~
,--, oo
tt)
eel
~
t'~
P~
o
e,i + e4
¢q
+
oo
¢'4
z
e,I
.-~
Diopsides Ma o 201 SiO2 TiO2 Al203 Cr203 FeO* MnO MgO CaO Na20 K20 Total Si Ti Al Iv Al vl Cr Fe3+ Fe 2+ Mn Mg Ca Na K Total Oxygens
55.20 0.04 0.29 -0.40 -19.62 24.18 0.18 -99.91 1.980 0.002 0.013 --. 0.013 -1.049 0.928 0.013 -3.998 6
Melilites Z R 248
52.25 0.40 2.84 0.10 2.93 0.06 16.41 24.50 0.18 0.08 99.75
36.85 0.06 38.49 0.05 0.33 0.06 0.03 22.90 0.04 -98.81
1.916 0.001 0.084 0.039 -.
. 0.090 0.002 0.896 0.963 0.013 0.004 4.018 6
1.544 0.003 -1.902 0.003 . 0.013 0.003 0.003 1.028 0.005 -4.504 7
37.68 -31.58 -0.86 0.02 0.05 24.88 0.01 -95.08
38.45 0.07 30.41 0.06 0.37 -0.03 23.32 0.04 0.005 92.78
1.653 --1.635 --
1.723 0.003 -1.605 0.003
0.032 0.007 0.032 1.171 --4.530 7
0.013 -0.003 1.120 0.003 -4.473 7
.
246
ZHOU GUOQING Table 5. Analysis and calculated formula of crossite from aragonitc-jadeite glaucophane schist at Xiwan
SiO2 TiO2 AI203 Cr203 Fe~O3 FeO* NiO MnO MgO CaO Na20 K20 LOI Total
Xiwan Crossite
Xi Winchite
57,46 0.10 6.41 . 4.90 3.90 . 0.17 14.09 3.61 6.27 0.28 2.14 99.33
56.49 0.14 1.17 .
12-2 Magnesioarfvedsonite
Xi-12 Cummingtonite
56.67 0.06 0.56 .
54.50 0.01 0.75
. . I 1.50
.
.
. 16.83
12.22 . 0.45 17.68 2.28 5.65
.
JX Crossite
12 Glaueophane
50.83 0.11 3.40 0.05 . 7.24 0.04 0.13 24.02 10.28 2.76 . . 98.86
60, I 7 -2.70
59.38 -5.20
-
-
-
-
. 7.77 --20.92 1.29 7.14
12.96 --14.87 2.24 5.35
95.57
0.70 21.27 0.83 1.00 . . 95.89
99.99
100.00
Si Ti A1w A1vl
7.95 0.01 0.04 1.01
7.99 0.02 -0.19
8.19 0.01 -0.09
7.91 -0,09 0.03
7.15 0.01 0.56 --
7.88 -0.12 0.29
7.69 -0.31 0.48
Fe 3+ Fe 2+
0.51 0.45
-1.36
-1.48
-2.04
0.02 0.84
-0.85
-1.40
0.02 2.90 0.53 1.68 0.05 15.15 24
0.07 4.09 0.26 1.85 . 15.83 23
0.05 3.81 0.36 1.58
0.08 4.61 0.13 0.28 . 15.17 23
0.03 5.03 1.55 0.75
-4.08 0.18 1.81
-2.87 0.31 1.34
15.21 23
14.40 23
Mn Mg Ca Na K Total Oxygens
0.54 19.40 1.70 6.79 . . 97.73
.
J2x- 12 Szechenite
. .
. .
.
. 15.56 23
. .
.
.
15.96 23
cut the melilite. Therefore it is possible that they may occur in the Proterozoic era as the NEJXO, but the high-temperature metamorphism is earlier than the highpressure metamorphism, which I termed as "superimposition metamorphism" or "Remetamorphism". I think that Miyashiro's "paired metamorphism" theory only suits the collision mechanism of the convergent plates at the adjacent space in the same age, i.e. the high-T and high-P of paired metamorphism occur at the continent side of the collision zone and the ocean side, respectively, during the period when the collision is
The compositions of the crossite and other amphiboles, lawsonite, jadeite and aragonite are shown in Tables 5 and 6, respectively. It is obvious that the compositions of these minerals vary on a larger scale and are largely controlled by the original rocks and the CaO of the lawsonite in the marble, SiO2 in melilite and jadeite are slightly more than the common values. It is interesting to note that the crossite may come from the earlier tremolite in the hornstone, the schistose lawsonite marble occurs along a compression fault within the melilite marble and the lawsonite
Table 6. Microprobe data and calculated formulae of aragonite, jadeite and lawsonite Lawsonite
Jadeite Jx-05: X-12
Z R Z 203-5 SiO2 TiO2 A1203 Cr~O3 FeO* MgO CaO MnO NiO Na20 K20 Total Si Ti AI Fe 2+ Mg Ca Mn Na K Total Oxygens
40,58 0,02 33.78 -0.24 0.19 25.07 0.06 --0.02 99.96 1.928 0.001 1.891 0.009 0.013 1.275 0.002 -0.001 5.120 8
40.35 0.03 34.01 -0.18 0.05 25.28 0.05 -0.03 0.01 99.99 1.918 0.001 1.906 0.007 0.003 1,288 0,002 0.003 0.001 5.129 8
40.54 . 31.69 -0.17 2.71 24.57 . --0.33 100.01
69.20 .
. 22.19 0.03 0.06 0.02 0.28
.
. 0.01 7.89 0.30 99.98
1.933 2.233 . . . 1.781 0.844 0.006 0.002 0.192 0.001 1.254 0.010 . . . -0.494 0.020 0.012 5.186 3.597 8 6
68.52 . 26.83 0.02 0.02 0.03 0.54 . -4.01 0.01 99.98 2.175 . 1,004 0.001 0.001 0.018 . 0.247 -3.446 6
Aragonite Jx-05 Xi12-2 67.67 . 17.64 -0.05 0.05 0.17 -7.49 -93.07 2.330 0.717 -0.002 0.006 0.501 -3.556 6
0.06 0.04 0.07 0,17 0,03 52.42 0.01 0.04 0.16 -53.00
0.06 0.04 0.02
0.15 0.19 0.04
0.I0 0.01 55.50 0.03
0.13 0.02 54.45 0.12
0.06
0.05
55.82
55.15
Features of northeastern Jiangxi ophiolite (NEJXO) advancing, but the superimposition metamorphism is more suitable for the "open(tension)-closure (compression) mechanism" of the smaller ocean basins located at continental margins, that is, "superimposition metamorphism" means that the high-temperature metamorphism resulting from magmatism during the "open" (early) stage is superimposed on the highpressure metamorphism resulting from tectonism during the "closure" (late) stage at the same place but at different ages. Both the superimposition metamorphism and paired metamorphism may together be termed as "bimetamorphism". CONCLUSIONS Here, the author has described and discussed in detail both the Proterozoic metamorphic peridotite and superimposition metamorphism. Although a lot of further work is required, we can say that older metamorphic peridotites must be different from those of younger ages in some aspects. Paired metamorphism only affects one side of the plate and can better explain metamorphism due to plate collision. Superimposition metamorphism suggested here appears to be able to explain metamorphism of small oceanic basins located at continental margins. REFERENCES Bureau of Geology and Mineral Resources of Jiangxi Province. 1984. Regional Geological Analyses of Jiangxi Province. Geological Memoirs, Ministry of Geology and Mineral Resource, People's
247
Republic of China. Regional Geology 2nd. Geological Publ. House, Beijing. Coleman, R. G. 1977. Ophiolite: Ancient Oceanic Lithospere? Springer, Berlin. Coleman, R. G. 1984. The diversity of ophiolites. Geol. Mijnbouw 63, 141-150. England, P. C. and Richardson, S. W. 1977. The influence of erosion upon the mineral facies of rocks from different metamorphic environments. J. Geol. Soc., Lond. 134, 201-213. Gass, I. G. 1982. Ophiolite. Scient. Amer. 247. Guo, L. Z., Shi, Y. S. and Ms, R. S. 1980. The geotectonic framework and crustal evolution of South China. In: Scientific Papers on Geology for International Exchange, Prepared for the 26th International Geological Congress. Tectonic Geology and Geological Mechanics. Geological Publ. House, Beijing. Guo, L. Z., Shi, Y. S. and Ma, R. S. 1983. On the formation and evolution of the Mesozoic-Cenozoic active continental margin and island arc tectonics of the western Pacific Ocean. Acta Geol. Sin. 1, ll-21. Huang, T. K., Chang, C. K., Chang, C. M. and Chen, K. M. 1965. On Eugeosynclines and Miogeosynclines of China and their Polycyclic Development. Collected works of geological sciences, C series. Ministry of Geology, People's Republic of China. Li, J. L., Zhang, F. Q. and Wang, S. X. 1983. REE distribution patterns of some rocks in the Proterozoic ophiolite in Yanbian, Sichuan. Petrol. Res. 3, 35-44. Miyashiro A. 1961. Evolution of metamorphic belts. J. Petrol. 2, 22-311. Nicolas, A. and Jackson, E. D. 1972. Repartition en deux provinces des peridotites des chains alpines longeant la M6diterranee: implications geotectoniques. Bull. Soc. Min. Petr. 52, 479-495. Nisbet, E. G. and Fowler, C. M. R. 1982. The thermal background to metamorphism sample two dimensional conductive models. Geosci. Can. 9, 161-164, 208-214. Ren, J. S., Jiang, C. F., Zhang, Z. K. and Qin, D. Y. 1980. The tectonic evolution of China. Scientific Papers on Geology for International Exchange, Prepared for the 26th International Geological Congress. Tectonic Geology and Geological Mechanics. Geological Publ. House, Beijing. Shu, L. S. and Li, Y. J. 1987. On terrane tectonics of North JiangXi. Geol. JiangXi 1, 31-37. Zhu, X., Huang, C. K., Rui, Z. Y., Zhou, Y. H., Zhu, X. J., Hu, C. S. and Mei, Z. K. 1983. Porphyry Copper Deposits at Dexin Jiangxi. Geological Publ. House, Beijing.