Petrogenesis and metallogenesis of the Yanshanian adakite-like rocks in the Eastern Yangtze Block

Vol. 46 Supp. SCIENCE IN CHINA (Series D) March 2003

Petrogenesis and metallogenesis of the Yanshanian adakite-like rocks in the Eastern Yangtze Block

WANG Qiang (王强), ZHAO Zhenhua (赵振华), XU Jifeng (许继峰), LI Xianhua (李献华), BAO Zhiwei (包志伟), XIONG Xiaolin (熊小林) & LIU Yimao (刘义茂)

Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China Correspondence should be addressed to Wang Qiang (email: [email protected])

Received September 20, 2002

Abstract Many of the Yanshannian intermediate-acid intrusive rocks related to Cu-Au mineraliza-tion in the Eastern Yangtze Block are characterized by high Al2O3, Sr contents, while low in Y, Yb contents, thus with high Sr/Y, and La/Yb ratios, and variational isotope signatures in particular, e.g. ε Nd( t ) = −11.92—1.96, (143Nd/144Nd)i = 0.5120—0.5125, TDM = 0.70—1.71 Ga, (87Sr/86Sr)i = 0.7043—0.7076. The geochemical characteristics of these rocks suggest that: (1) these rocks are geo-chemically similar to adakite, which might have been stemmed from the partial melting of thick-ened basaltic lower crust due to basalt underplating; and (2) the high pressure (1.2—4.0 GPa) and high temperature (850—1150℃) surroundings of the lower crust favor both the fluid and ada-kite-like magma to generation. Not only can the adakite-like magma carry abundant fluid and Cu-Au ore-froming materials, but also can it bring them to the shallow part with ease and contrib-utes to the Cu-Au mineralization.

Keywords: adakite, Cu-Au metallogenesis, underplating, Yanshanian Period, Yangtze Block.

Adakite-like rocks (SiO2≥56%, high Sr/Y (≥40) and La/Yb (≥20), and stongly depleted in

Y (≤18×10−6) and Yb (≤1.90×10−6)) are among the hotspots of geological researches in the

last decade[1—6]. Adakite-like rocks can be related to some deep dynamic processes, such as oce-

anic crust subducting[1,5,6], basalt underplating[2—4,7], delamination of lower crust[7,8], and thus are geodynamically important. Recently, the Cu-Au mineralization related to adakite-like rocks has been kept a watchful eye on[9—12]. Some researches suggest that some adakite-like rocks can be closely related to Cu-Au mineralization and plenty of epithermal and porphyry deposits are found in the regions with extensive Cenozoic adakite-like magmatic activities, for instance, in Philippine and Papua New Guinea in the west Pacific Ocean, the west of the United States and Chile[9,11]. Based on their studies on the volcanic rocks in north Chile, Oyarzun et al.[12] indicated that the early stage “normal” intermediate-acidic igneous rocks, which show right dipping REE patterns without obvious HREE depletion, are closely associated with the small-sized porphyry deposits (e.g. Lomas Bayas); on the contrary, the late stage adakite-like rocks are closely associated with the superlarge porphyry Cu deposits (e.g. Chuquicamata). Therefore, the adakite-like rocks are

Supp. MESOZOIC ADAKITE-LIKE ROCKS & METALLOGENESIS IN EAST CHINA 165

probably closely related to Cu-Au mineralization. However, the kin relationship between the rocks and Cu-Au mineralization needs farther investigation.

The middle and lower reaches of the Yangtze River and the Dexing regions of Jiangxi Prov-ince in the Eastern Yangtze Block are important Cu-Au-Fe mineralization areas in China. Many geologists have done a lot of research work in this region[13—19]. This study shows that the Cu-Au-Fe deposits in the Eastern Yangtze Block are closely related to the widespread Yanshanian igneous rocks. Although lots of work concerning these rocks and the relationship between the rocks and metal deposits have been done, the geodynamic setting (continental arc or rift?) in which these igneous rocks and related Cu-Au-Fe deposits were produced have been bewildering. Recently, some researches[20—22] show that many Yanshanian igneous rocks in this area are geo-chemically similar to adakite. This paper will emphasize the characteristics of some Yanshanian igneous rocks related to Cu-Au-Fe mineralization in the Eastern Yangtze Block and investigate the relationship between these rocks and mineralization.

1 Geological settings

There are many Yanshanian Cu-Au-Fe deposits in the middle and lower reaches of the Yang-tze River and Dexing of Jiangxi Province in the Eastern Yangtze Block (fig. 1). In the middle and lower reaches of the Yangtze River, many Cu-Au-Fe deposits are associated with igneous rocks: (1) in southeast Hubei Province, the representative deposits are Tieshan Fe-Cu and Tonglushan Cu deposits, and the related igneous rocks are the Tieshan and Tonglushan (Yangxin) quartz diorites, respectively; (2) in the Fengshan-Ruichang-Jiujiang area, the representative deposits are Feng-shandong Cu-Mo and Wushan Cu deposits, and related igneous rocks are Fengshandong grano-

Fig. 1. The sketch diagram showing the distribution of the intrusive rocks related to Cu, Fe, Au minerali-zation in the Eastern Yangtze Block.

166 SCIENCE IN CHINA (Series D) Vol. 46

diorite porphyry and Wushan granodiorite respectively; (3) in the Huaining-Lujiang area, the representative deposits are Yueshan Cu-Fe and Shaxi Cu-Au deposits and Zongpu Fe mineralizations, and the related igneous rocks are Yueshan diorite, Shaxi quartz diorite porphyrite and Zongpu granodiorite, respectively; (4) in the Tongling area, the representative deposits are Tongguanshan, Shizishan and Dongguashan Cu deposits related to the Tongguanshan and Shizishan diorites, Dongguashan quartz diorite, respectively; and (5) in the Ningzhen area, the representative deposits are Anjishan Cu-Mo deposit which are related to the Anjishan granodiorite. In the Dexing area, the representative deposits are Tongchang and Zhushahong Cu, and Fujiawu Cu-Mo deposits, which are correspondingly related to the Tongchang, Zhushahong and Fujiawu granodiorite porphyries. The acreage of every intrusive body ranges from 0.06 to 200 km2, the largest is Tonglushan (Yangxin) body (200 km2), and the smallest is Zhushahong body (0.06 km2). The deposits were produced approximately in accordance with those of the related intrusive rocks, most of the intru-sive rocks were produced in the late Yanshan Peroid (140—100 Ma)[13—15,18] except those in the Dexing area which were formed in the early Yanshan Peroid (184 Ma)[16,17].

2 Petrochemistry

The major element contents of the representative intrusive rocks related to Cu-Au-Fe mineralization in the Eastern Yangtze Block are listed in table 1. The characteristics of major elements are as follows: (1) in the An-Ab-Or diagram (fig. 2), most of the samples are plotted in the granodiorite-trondhjemite-tonalite area, some in the area of granite; (2) SiO2≥54%, mainly of

middle potassic calc-alkaline and high potassic calc-alkaline (fig. 3(a)); (3) high in Al2O3 (≥

14.5%) (fig. 3(b)); (4) rich in Na (Na2O≥3.0%, mostly ＞3.5% ) (fig. 3(c), table 1); (5) MgO =

1.52—2.91%, Mg# (100×Mg2+/(Mg2++Fetotal)) = 37—53 (table 1); and (6) they are mostly plotted in the field of the melt compositions of metabasalt or amphibolite in the melting experiment under high pressure (1.0—4.0 GPa) and high temperature (850—1150℃)[23—25] (fig. 3(a), (b), (c)).

Fig. 2. The Ab-An-Or diagram of igneous rocks. 1, The ada-kite resulting from the melting of subducting slab ([1, 5, 6] andcited references within); 2, the adakite-like rocks resulting fromthe melting of thickening basaltic lower crust; 3, the intrusiverocks related to Cu mineralization in the Dexing area of JiangxiProvince (bodies: Tongchang, Fujiawu and Zhushahong) ([16,17] and this study); 4, the intrusive rocks related to metal depos-its in Fengshan-Ruichang-Jiujiang area (bodies: Fengshandongand Wushan) ([19] and this study); 5, the intrusive rocks relatedto metal deposits in the Southeast of Hubei Province (bodies:Tieshan, Tongshankou and Tonglushan) ([18] and this study); 6,the intrusive rocks related to metal deposits in the Huain-ing-Lujiang area (bodies: Shaxi, Yueshan and Zongpu) ([13, 15,24] and this study); 7, the intrusive rocks related to metal depos-its in the Tongling area (bodies: Tongguanshan, Shizishan andDongguashan) ([13, 14] and this study); 8, the intrusive rocksrelated to metal deposits in the Ningzhen area (body: Anjishan)([13, 14, 27] and this study).


Table 1 The major and trace elements contents of the representative intrusive rocks related to Cu-Au-Fe mineralization in the

Eastern Yangtze Block Intrusive

body Tieshan Tong-lushan Fengshd Fengshd Shaxi Yueshan Zongpu Tonggsh Anjishan Tong-

chang Fujiawu Zhushh

Rock Qδ Qδ γδπ γδπ Q δμ Q δ Q δ Q δ γδπ γδπ γδπ γδπ Deposit Fe-Cu Cu Cu Cu Cu, Au Cu, Fe Fe Cu Cu Cu Cu, Mo Cu Sample 00HB-1 00HB-8 F-2 F-12 98LZ-036 00YS-5 ZP-2 AC-16 28 01TC-03 01FJW-3 G-83-175 SiO2 63.77 63.68 64.19 65.57 57.59 60.56 63.95 62.61 65.05 65.28 68.08 64.67 TiO2 0.62 0.55 0.53 0.60 0.59 0.69 0.49 0.57 0.47 0.53 0.44 0.49 Al2O3 15.81 15.92 14.94 14.94 16.75 15.88 16.51 16.83 14.98 15.18 15.33 14.97 Fe2O3 1.53 2.39 1.79 0.68 3.84 2.63 1.80 2.25 2.56 0.99 1.44 1.55 FeO 2.52 2.27 2.28 2.60 3.87 2.85 2.00 1.78 2.22 3.02 2.18 2.60 MnO 0.06 0.08 0.12 0.05 0.04 0.10 0.07 0.06 0.04 0.12 0.03 0.11 MgO 1.62 1.44 2.12 1.95 2.51 2.91 1.89 1.66 1.52 2.43 1.82 2.35 CaO 3.99 4.98 3.64 3.18 5.24 4.76 4.49 5.20 4.28 2.24 1.42 4.50 Na2O 4.84 4.35 3.53 3.81 4.37 4.59 4.98 4.91 4.38 3.51 3.46 3.57 K2O 3.30 2.75 3.46 3.60 1.88 3.24 2.19 2.52 2.98 2.10 3.38 3.30 P2O5 0.24 0.26 0.24 0.26 0.35 0.39 0.22 0.28 0.26 0.08 0.10 0.26 H2O 0.81 0.45 1.27 1.19 1.92 0.93 0.89 0.40 − 2.25 1.95 1.23 CO2 0.51 0.61 1.62 1.19 0.77 0.09 0.14 0.14 − 1.53 0.05 0.07 Σ 99.62 99.73 99.73 99.62 99.72 99.62 99.62 99.21 98.74 99.26 99.68 99.67 Mg# 43 37 49 52 38 50 48 44 37 53 48 51 Na2O/K2O 1.47 1.58 1.02 1.06 2.32 1.42 2.27 1.95 1.47 1.67 1.02 1.08 Cr 25.67 6.98 62.94 38.00 12.93 20.90 25.49 21.40 22.00 59.68 32.14 56.22 Ni 14.46 5.83 36.99 22.92 6.91 13.93 11.93 8.66 12.20 21.02 13.46 23.80 Co 7.93 9.45 12.48 15.04 11.35 15.68 9.13 5.16 10.50 10.37 7.46 10.37 Sc 4.98 6.35 9.54 9.78 11.20 9.71 6.19 N.D. 5.10 8.28 7.64 9.28 V 68.96 70.87 75.46 80.51 134.35 127.50 64.59 71.46 69.20 76.30 67.72 74.14 Rb 85 93 109 97 51 75 38 61 44 89.06 61.6 103.7 Ba 1016 734 810 704 357 1124 1339 934 1591 3958.6 1283.4 1465 Sr 1609 961 686 626 1009 1535 1566 1011 906 1833 573.8 817.6 Ta 0.75 1.03 0.76 0.92 0.21 0.38 0.37 N.D. 1.10 0.74 0.77 0.86 Nb 13.42 18.08 10.87 12.62 4.18 8.76 7.30 7.00 14.50 8.56 8.09 10.37 Hf 5.07 4.04 3.26 4.39 2.15 3.44 3.35 N.D. 2.50 2.79 4.04 1.59 Zr 165.55 130.59 101.73 139.40 70.69 92.81 123.90 187.00 69.70 97.10 153.50 41.38 Y 13.26 17.04 13.57 11.23 12.43 13.79 11.16 14.60 9.20 7.68 6.20 13.43 U 2.24 3.24 2.58 2.30 0.83 3.19 5.06 N.D. 2.60 4.11 2.40 5.08 Th 7.53 13.44 10.95 11.29 3.62 12.50 1.69 6.91 11.90 17.71 16.52 20.54 δSr 2.06 1.10 1.06 0.81 1.92 1.64 2.90 1.45 1.34 4.41 1.80 1.28 Sr/Y 121 56 51 56 81 111 140 69 98 239 93 61 La 42.58 51.00 37.80 44.12 26.37 51.86 28.49 37.00 40.10 31.46 23.40 41.26 Ce 84.78 97.74 69.44 86.75 54.92 101.79 56.68 75.50 78.10 48.64 36.98 72.10 Pr 9.32 10.31 7.72 9.00 6.41 10.78 7.03 7.89 7.76 5.20 3.98 8.25 Nd 35.44 37.78 29.79 33.53 25.68 42.70 26.12 32.05 26.90 16.29 12.77 27.12 Sm 6.41 6.83 5.46 5.22 5.27 6.93 4.23 5.82 4.20 2.54 2.03 4.32 Eu 1.73 1.78 1.44 1.46 1.63 1.93 1.12 1.65 1.39 0.54 0.59 1.09 Gd 4.40 4.93 4.15 3.99 4.17 5.00 2.73 4.51 3.40 1.94 1.50 3.10 Tb 0.55 0.63 0.53 0.46 0.53 0.57 0.39 0.63 0.42 0.27 0.22 0.48 Dy 2.85 3.34 2.63 2.32 2.62 3.11 1.98 3.01 2.12 1.38 1.07 2.37 Ho 0.49 0.63 0.47 0.42 0.49 0.57 0.36 0.56 0.35 0.26 0.20 0.45 Er 1.29 1.88 1.39 1.09 1.26 1.43 0.96 1.46 0.94 0.70 0.55 1.18 Tm 0.18 0.25 0.16 0.14 0.15 0.21 0.14 0.22 0.12 0.12 0.08 0.17 Yb 1.09 1.71 1.13 1.04 1.07 1.30 0.94 1.37 0.94 0.80 0.60 1.19 Lu 0.15 0.24 0.18 0.14 0.17 0.16 0.15 0.21 0.14 0.14 0.11 0.19 ΣREE 191.27 219.04 162.27 189.70 130.72 228.32 131.31 171.88 166.46 110.27 84.09 163.28 δEu 0.99 0.94 0.92 0.98 1.06 1.00 1.01 0.98 1.12 0.75 1.04 0.91 La/Yb 39 30 33 42 25 40 30 27 43 39 39 35

The sample of Tonggshan intrusive rocks is from Li (2000), other samples are from this study. γδπ, Granodiorite, Qδμ, quartz diorite porphyruite, Qδ, quartz diorite, δ, diorite.


Fig. 3. Comparison of the intermediate-acid igneous rocks and the melt compositions of amphibolites (or metabasalts) un-der high pressure (1.0—4.0 GPa) and temperature (850—1150℃)[23—25]. (a) SiO2-K2O diagram; (b) SiO2-Al2O3 diagram; (c) SiO2-Na2O diagram; (d) SiO2-CaO diagram. The igneous rocks related to Cu-Au mineralization from other field in the world are cited from [9—12]. 1—8 are the same as that of fig. 2, 9 represents the melt compositions of basaltic rocks under 1.0—4.0 GPa pressure and ＞850℃ temperature; 10 represents the intrusive rocks related to Cu-Au mineralization in other areas in the world.

3 Trace elements characteristics

The trace elements of the intrusive rocks related to Cu-Au-Fe mineralization in the Eastern Yangtze Block are listed in table 1. The characteristics of trace elements are as follows: (1) ΣREE = (84.09—228.32)×10−6, showing steep rare earth elements (REE) patterns without obvious Eu

anomaly, i.e. rich in light REE and strongly depleted in heavy REE (La/Yb = 25—42) (fig. 4(a)); (2) depleted in Nb, Ta, Zr, Hf, and Ti without obvious Sr anomalies on the spide profiles (fig. 4(b), 5(a), table 1); (3) high in Sr (≥400×10−6, can be up to 2800×10−6) (fig. 5(a)); (4) low in Y

(mostly≤18×10−6, fig. 5(b)) and Yb (mostly≤1.9×10−6, fig. 5(c)); and (5) in the Sr/Y-Y dia-gram (fig. 5(b)), except those of intrusive rocks in the Tongling area which are plotted in the field near the boundary of adakite and ‘normal’ arc andesite-dacite-rhyolite, all the other samples are plotted in the adakite field.


Fig. 4. The chondrite normalized rare earth element patterns (a) and diagram of middle oceanic ridge basalts-normalized in-compatible elements (b) of the intrusive rocks related to mineralization in the Eastern Yangtze Block.

Fig. 5. The diagrams of trace elements of the intrusive rocks related to mineralization in the Eastern Yangtze Block. (a) Sr-δ Sr (= 2×SrN/(CeN+NdN)) diagram, samples of the felsic igneous rocks resulted from the melting of basaltic lower crust under low (＜1.0 GPa pressure are cited from [26]; (b) Y-Sr/Y diagram) [1]; (c) SiO2-Yb diagram; (d) SiO2-δ Eu (= 2×EuN/(SmN+GdN))[27]. The symbols are the same as in fig. 3.

4 Isotopes

The Sm-Nd and Rb-Sr isotopic compositions of the intrusive rocks related to Cu-Au-Fe min-eralization in the Eastern Yangtze Block are listed in table 2. The main characteristics of these


isotopic compositions are as follows: (1) ε Nd(t) = −11.92—1.96, (143Nd/144Nd)i = 0.5120—0.5125,

TDM = 0.70—1.71 Ga, and (87Sr/86Sr)i = 0.7043—0.7076; (2) the highest ε Nd(t) and lowest (87Sr/86Sr)i and TDM values are found in the Tongchang intrusive rocks whilst the highest (87Sr/86Sr)i and TDM but lowest (143Nd/144Nd)i values are found in the Tongling intrusive rocks.

Table 2 Sm-Nd and Rb-Sr isotopic compositions of some intrusive rocks related to mineralization in the East Yangtze Block

Number 1 2 3 4 5 6 7 8 9 10 11

Intrusive body Tieshan Tong-lushan

Feng-shandong Shaxi Yueshan Tongguanshan Tong-

chang Sample 00HB-1 00HB-8 F-12 98LZ036 98LZ004-1 98LZ005 AYS-1 − AI-4-2 AI-4-10 D01 t/Ma 134 130 130 130 126.8 126.8 136 136 137 137 184 Sm 6.05 11.62 5.35 5.58 4.85 4.99 5.93 8.75 5.50 5.00 Nd 35.69 69.98 33.78 28.55 26.85 43.95 32.47 53.02 31.51 28.42 147Sm/144Nd 0.1025 0.1004 0.0958 0.1182 0.1093 0.1136 0.1105 0.0998 0.1055 0.1063 0.0972 143Nd/144Nd 0.512126 0.512157 0.512256 0.512368 0.512305 0.512312 0.512143 0.512166 0.511949 0.511946 0.512619

σ 9 5 10 15 12 14 5 30 8 5 9 (143Nd/144Nd)i 0.5120361 0.5120716 0.5121745 0.512268 0.512214 0.512218 0.512045 0.512077 0.511854 0.511851 0.5125019

εND(t) -8.38 -7.79 -5.78 -3.98 -5.08 -5.02 -8.16 -7.53 -11.85 -11.92 1.96 TDM/Ga 1.40 1.33 1.16 1.25 1.23 1.27 1.48 1.32 1.69 1.71 0.70 Rb 87.86 92.81 97.97 49.95 158.7 104.4 58.25 103 73.19 80.77 Sr 2524 915.1 605.4 930 471 726 1589 1208 1225 1032 87Rb/86Sr 0.1008 0.2935 0.4684 0.1555 0.9755 0.4164 0.1061 0.2465 0.1729 0.2265 87Sr/86Sr 0.706513 0.706377 0.70776 0.705464 0.706918 0.706087 0.70664 0.70696 0.707895 0.707666

σ 15 5 6 11 9 10 20 4 8 7 (87Sr/86Sr)I 0.7063 0.7058 0.7069 0.7052 0.7052 0.7053 0.7064 0.7065 0.7076 0.7072 0.7043

1—6, This study; 7—10, from Chen et al. (1993), 11, from Zhu et al. (1990).

5 Discussion and conclusions

5.1 Petrogenesis The intrusive rocks related to Cu-Au-Fe mineralization in the Eastern Yangtze Block are

geochemically similar to adakite (fig. 5(b)), such as SiO2≥54%, high Al2O3(≥14.5 %), sodium

(Na2O≥3.0%), and Sr (≥400×10−6, the highest up to 2800×10−6), low Yb (generally≤

1.90×10−6, except a few samples from the intrusive rocks in the Tongling area ≥1.90×10−6) and

Y (≤18×10−6, except a few samples of the intrusive rocks in the Tongling area ≥18×10−6),

high La/Yb (25—42), and depleted in Nb, Ta and Ti, which resemble those of the ‘normal’ arc andesite-dacite-rhyolite. Adakite is originally thought to result from the melting of subducting young (≤25 Ma) basaltic crust when they are transformed eclogite[1]. However, (143Nd/144Nd)i

(0.5119—0.5125 and (87Sr/86Sr)i (0.7043—0.7076) values of the adakite-like rocks in the Eastern

Yangtze Block are obviously different from those ((143Nd/144Nd)i＞0.5130 and (87Sr/86Sr)i＜

0.7040 of middle oceanic ridge basalt (MORB). Therefore, they could not be stemmed from the

melting of subducting basaltic oceanic crust. But isotopic compositions ((

)143Nd/144Nd)i = 0.5125,

(87Sr/86Sr)i =0.7043) of the Tongchang intrusive rocks are similar to those ((143Nd/144Nd)i≥


0.5125, (87Sr/86Sr)i≤0.7050) of adakite-like rocks resulting from the melting of subducting sedi-ment-bearing basaltic oceanic crust[5]. As adakite-like rocks can also result from the melting of flat subduction and old (≥25 Ma) oceanic crust[28], and Zhou and Li[29] recently emphasized the Yan-shanian flat subduction of the Izanaqi Plate probably took place in the southeast of China, some adakite-like rocks in the Eastern Yangtze Block may also be possibly derived from the melting of the flat subduction oceanic slab. But further investigations need to be done.

The melting of thickened basaltic lower crust perhaps is an important alternative mechanism of adakite-like rocks production[2—4]. The adakite-like rocks related to Cu-Fe-Au mineralizations in the Eastern Yangtze Block were also probably formed by the melting of basaltic lower crust as evidenced by: (1) the adakite-like rocks, with the exception of the Tongchang intrusive rocks, re-lated to metallogenesis in the Eastern Yangtze Block have low (143Nd/144Nd)i (0.5119—0.5123) and

high (87Sr/86Sr)i (0.7052—0.7076), which are obviously different from those of the adakite-like rocks derived from the melting of basaltic or sedimentary materials-bearing oceanic crust. (2) The ada-kite-like rocks derived from the melting of basaltic lower crust and the adakite-like rocks in the Eastern Yangtze Block have higher K2O content than that resulting from the melting of subducting oceanic crust. This is probably due to higher K2O content of the basaltic lower crust in the conti-nent than those of basaltic oceanic crust [21,30]. The Shaxi intrusive rocks were probably formed by the melting of underplating K-rich basaltic lower crust[20]. This supports that the high-K ada-kite-like rocks were probably stemmed from the melting of high-K source rocks. (3) Mg#(≤53) (table 1) of the Yanshanian adakite-like rocks in the Estern Yangtze Block (table 1) is lower than that (58—72)[5] of the typical adakite resulting from the melting of subducting oceanic crust (for example, Aleutian islands in the United States, Cerro Pampa in Argentina and Cook Island in Chile). This suggests that the Yanshanian adakite-like rocks in the Estern Yangtze Block were most probably related to the crust melting[30]. (4) The compositions of the Yangshanian ada-kite-like rocks in the Eastern Yangtze Block accord with melt compositions of metabasalt or am-phibolite in the melting experiment under high pressure (1.0—4.0 GPa) and high temperature (850

—1150℃)[23—25] (fig. 3). (5) The compositions of the Yangshanian adakite-like rocks in the East-ern Yangtze Block are different from those of the intrusive rocks resulting from the melting of basaltic lower crust under ＜1.0 GPa pressure in the Cascades of North America[26] (fig. 5(a), (b),

(c)), for example, the former has higher δ Sr(= 2×SrN/(CeN+NdN)), δ Eu (= 2×EuN/(SmN+GdN)), Sr/Y and Sr content (fig. 5(a), (b), (d)), and lower Y and Yb contents (fig. 5(b), (c)) than those of the latter. It indicates that plagioclase probably made default while garnet was residual mineral in the source of the former. Therefore, the adakite-like rocks in this study were most probably stemmed from the melting of basaltic lower crust under high pressure and temperature. Melting experiments[23—25] suggested that garnet would not be equilibrated with the melt originating from the melting of basaltic rocks until the pressure beyong 1.2 GPa (namely, corresponding to 40 km in the deep crust). That is


to say, the garnet must be residual mineral during adakite-like magma producing. Thus, the exis-tence of garnet in the residual phase is the prerequisite for adakite-like magma generation. During the melting of the lower crusal rocks under high pressure and temperature, the dehydration of hornblende and decomposition of plagioclase lead to the production of adakite-like magma and leave garnet-bearing residua. Samples from the Tongling area showing geochemical features (fig. 5(b), (c)) different from those of the typical adakite-like rocks probably result from the mixing of magma related to the melting of thickened lower crust and mantle-derived magma[31].

The melting experiments[23—25] suggested that basaltic rocks could not be melted until the

temperature is up to 850—1150℃, that is to say, the melting of basaltic rocks in crust needs un-usual heat source. Guffanti et al.[32] thought that basalts underplating in the lower crust were the most probable heat source that lead to the melting of the lower crustal rocks. Atherton et al.[2] suggested that basaltic magmas newly underplating not only made crust grow vertically, but also provided heat for the melting of basaltic lower crust. The adakite-like magmas could be produced by the melting basaltic lower crust under the conditions of ＞40 km thickness of the crust and heat provided by underplat-ing basaltic magmas. Therefore, the adakite-like rocks in the Eastern Yangtze Block most proba-bly result from the melting of basaltic lower crust whilst Meszoic basaltic magmas underplating in the Southeast of China provided the necessary heat[29].

5.2 Adakite-like magma and related ore mineralization

5.2.1 Dynamic settings of the ore mineralization. The dynamic settings of the Yanshanian Cu-Au-Fe mineralization in the Eastern Yangtze Block is still open to debate. Was it continental margin arc or swell? Was it rift or concavity? In 1972, Sillitoe[33] studied the relationship between porphyry Cu deposits and subduction belts of plates, proposed that porphyry magma and main minerogenic elements came from the subducting oceanic plate, and emphasized the contribution of slab melts to porphyry and metallogenic metal materials. Some recent researches[10—12] sug-gested that adakite-like rocks usually associated with Cu-Au-Mo epithermal and porphyry depos-its, and metallogenic metal materials could come from the adakite-like magma. However, not all porphyry Cu deposits were formed in the arc setting and slab melts took part in the formation of deposits. For example, the ore-forming age of some porphyry Cu deposits did not accord with that of plate subduction, and the time gap can be up to few tens of million years[34]. Hollister[35] thought that the subduction belt of plate was not the prerequisite for porphyry Cu mineralization, parent magmas of porphyry might not always come from the melting of oceanic crust in the sub-duction belt, and porphyry Cu mineralization may take place when a melting zone is reached in the lower crust or supramantle with a deep fault cut though. The materials of porphyry could not always come from originally enriched oceanic crust. For example, Arizona porphyry Cu deposits, although they occurred in continental maigin arc, they were not directly related to the mantle wedge metasomatised by the fluid from subducting oceanic crust or magmas resulting from the


slab melting. Both basalt derived from the wedge underplating and the melting of thickened old lower crust controlled the formation of the Arizona deposits, and magmas of metallogenic por-phyry originated from the melting of basaltic lower crustal rocks[36]. Thiéblemont et al.[9] reckoned that some Cu-Au-Mo deposits associated with adakite-like rocks were formed in the intraplate setting. Some recent researches suggested that some deposits were related to thickened continental crust, thus, probably were originated from the melting of thickened lower crust[37]. Most of Yan-shanian igneous rocks related to the mineralizations in the Eastern Yangtze Block are adakite-like, and were probably related to the melting of thickened ＞40 km lower crust. Therefore, the

Cu-Au-Fe deposits in the Eastern Yangtze Block were probably formed in the thickened continen-tal crust setting.

( )

5.2.2 Possible factors controlling mineralizations. Adakites resulting from either the melting of subducting oceanic crust or the melting of thickened basaltic lower crust are products of high pressure (1.2—4.0 GPa) and temperature (850—1150℃) magmatism[23—25]. In addition, the igne-

ous rocks in the Eastern Yangtze Block and other areas[9—12] have similar major and trace element compositions (figs. 3 and 5(b), (c), (d)), and most of them resemble the adakites or adakite-like rocks and the melts of metabasalt or amphibolite in the melting experiment under high pressure and high temperature (figs. 3 and 5). The Cu-Au mineralization associated with adakite or adakite-like rocks are probably related to the following factors:

(1) High temperature. Some researches showed that the homogenization temperature of the fluid inclusions in minerals from some epithermal and porphyry Cu deposits (for example, Dinkidi Cu-Au porphyry deposit, Philippines) could be up to 600—900℃[38]. The inclusion investigations on the Yanshanian intrusive rocks related to Cu-Au mineralization, and enclaves in the Tongling area showed that the temperature of melt inclusions is ＞1250—900℃; the temperature of melt or

fluid inclusions during the transitional stage (from magma to fluid) ranges from 900 to 750℃[39]. The existence of the high temperature inclusions indicates that these deposits were probably formed at high temperature and related to high temperature magmas. Therefore, one reason for the close association between Cu-Au-Fe mineralization and adakites or adakite-like rocks may be the high temperature (850—1150℃) under which the adakite-like magmas were formed since the high temperature condition is favorable for the migration of Cu-Au-Fe ore-forming materials.

(2) High pressure. Some experiments suggest that pressure can be one important factor controlling the migration of ore-forming materials. Khitarov et al.[40] found that the contents of Pb, Zn and Cu in melts were closely related to pressure and independent of temperature in the range of 700—900℃. Urabe[41] found that the partition ratios of metals decrease remarkably with increase of pressure in the experiment, which suggests that the metals are prone to exist in melt rather than vapor phase as the pressure increases. Even though the solubility in the silicate melt is high, H2O will represent only a small fraction of the fluid in the deep crust, H2O increasingly exsolves out of


the melt at low pressure until it dominates the fluid[42]. Under low pressure, metals, such as Cu, in magma can be scavenged by exsolving magmatic fluid[42]. Since the adakite-like magmas are produced at high pressure (1.2—4.0 GPa), they not only carry a lot of ore-forming materials, but also bring them from deep to shallow parts with ease.

(3) Fluid. Fluid plays an important role in the Cu-Au mineralization[42]. During the oceanic crust subduction, the oceanic crust will release a lot of fluid or be melted to produce adakite-like magma, which make the ore-forming materials enrich. Therefore, the Cu-Au deposits are easy to be formed in the setting of volcanic arc. The volcanic arc setting is favorable for Cu-Au mineralization because abundant fluid will be released and/or the adakite-like magma be generated as the oceanic crust subducting which will concentrate plenty of ore-forming materials. But, where does the fluid come in the lower crust circumstance?

In the lower crust, the adakite-like magmas may be produced through[23—25]:

Hornblende + plagioclase±quartz = garnet + pyroxene + adakite-like melt + neonatal albite

where hornblende will decompose and release a lot of fluid underunder high pressure (1.2—4.0

GPa) and temperature (850—1150℃). That is to say, the basaltic lower crustal rocks will release a lot of fluid during the transformation from amphibolite to eclogite, moreover, the fluid is favorable to both the production of adakite-like magma and the enrichment of Cu-Au ore-forming materials. However, only limited fluid will be released under thin crust (＜40 km ) and low temperature (＜

850℃ ) under which the hornblende is stable, thus less favorable for metallic mineralization.

(4) Rapid migration. The adakite-like rocks are unlikely derived from the fraction crystal-lization[1,2]. In the SiO2-δEu diagram (fig. 5(d)), the adakite-like rocks (including those from the Eastern Yangtze Block) related to Cu-Au mineralizations show high δ Eu which change approxi-mately horizontally with SiO2 increasing. Since the adakite-like magma was derived from the melting of basaltic rocks in the stable region of garnets and plagioclases are the main phases to be melted, the positive Eu anomaly or weakly negative Eu anomaly actually indicates that these magmas were probably primitive (no fractional crystallization of minerals)[43]. It is suggested that adakite-like rocks related to Cu-Au mineralizations did not possibly go through sufficient evolu-tion which resulted from the fleetly crystallization and ascendingof the magmas. Sillitoe[33] also noted that the little fractionated magmas were preferential for porphyry Cu-Au-Mo mineralization. So the fleet ascending and poor fractionation of the adakite-like magmas made ore-forming mate-rials migrate easily upwards.

In conclusion, since the adakite-like magmas were derived from the melting of basaltic rocks under high pressure and temperature which is favorable for the enrichment of the Cu-Au ore-forming materials with the help of the abundant fluids, the adakitic magmas are capable of dissolving and carrying plenty of Cu-Au ore-forming materials and fluid. When the magmas reach the shallow parts after they go through the crust fleetly, the ore-forming fluid will be released,


magmas be solidified as the pressure is decreased rapidly, and the further evolution of the ore-forming fluid may be ended in Cu-Au mineralization.

Acknowledgements We would like to thank Dr. Xu Wei and Zhu Ning, Sun Xingya and Dai Youfang for their zealous help in the field. Li Chaofeng, Liu Ying, and Hu Guangqian and Li Chaofeng are thanked for their enthusiastic help in sample analyzing. This work was supported by the National Climbing Program of China (Grant No. 95-Y-25), the National Natural Science Foundation of China (Grant Nos. 40273019 and 40172028), the Knowledge Innovation Program of the Chinese Acad-emy of Sciences (Grant Nos. KZCX2-102 and KZCX2-SW-117), the Major State Basic Research Program of China (Grant No. G1999043202).

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Petrogenesis and metallogenesis of the Yanshanian adakite-like rocks in the Eastern Yangtze Block

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