Identification and petrogenesis of Wuhema highly fractionated I-type granitoids in Qinghai Province from Songpan-Ganzi Terrane
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摘要:
为探讨松潘−甘孜地体内青海吾和玛花岗质岩石的成因类型与岩石成因，对其开展了岩相学、地球化学、锆石U−Pb年代学和Lu-Hf同位素等研究。吾和玛花岗质岩石被确定为高分异的I型花岗岩，其结晶年龄为216.6±4.3 Ma，为正长花岗岩，具有弱过铝质高钾钙碱性特征。岩石轻、重稀土元素分馏不明显 ((La/Yb)N <7)， 轻稀土元素轻微富集，重稀土元素分布平缓，具有明显的负Eu异常(δEu=0.08 ~ 0.43)，明显亏损Ba、Sr和高场强元素Nb 、Ta 、P 、Ti ，富集大离子亲石元素Rb、Th、U、K，εHf(t)值为−2.54～1.32。吾和玛花岗质岩石的母岩浆是岩石圈拆沉作用引起软流圈上涌，诱发中元古代地壳物质（类似于变杂砂岩的物质）部分熔融形成长英质母岩浆，后期又受到斜长石、钾长石、磷灰石等矿物分离结晶作用的控制，经历了高程度的分异形成的。
Abstract:In order to study the genetic type and petrogenesis of the Wuhema granitoids in the Songpan Ganzi Terrane, the petrography, geochemistry, zircon U−Pb chronology and Lu-Hf isotope have been studied in this paper. The Wuhema granitic rocks are identified as highly fractionated I-type granitoids with crystallization age of 216.6±4.3 Ma. The rocks are syenogranite, and have the characteristics of weakly peraluminous, high-K calc-alkaline. The fractionation of REE in the rocks is not obvious (La/Yb)N <7), light rare earth elements are slightly enriched, and heavy rare earth elements are gently distributed. The rocks have obvious negative Eu anomalies (δEu= 0.08 ~ 0.43), with obvious depletion of Ba, Sr and high field strength elements (HFSEs, e.g., Nb, Ta, P and Ti), and enrichment of large ion lithophile elements (LILEs, e.g., Rb, Th and U), εHf(t) values of the rocks range from -2.54 to 1.32. The primary magma of the Wuhema granitic rocks were formed by the upwelling of asthenosphere caused by the lithosphere delamination, and induced the partial melting of Mesoproterozoic crustal materials (similar to metagreywacke), then the magma went through a high degree of fractionation of plagioclase, potassium feldspar, apatite and other minerals.
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图 1 松潘−甘孜地体及邻区大地构造单元简图(a, 据Yin et al., 2000)和吾和玛地区地质简图（b）
Figure 1.
图 4 吾和玛花岗质岩石稀土元素球粒陨石标准化配分图(a)及微量元素原始地幔标准化蛛网图(b) (标准化值据McDonough et al.,1995)
Figure 4.
图 6 吾和玛花岗质岩石源区组成判别图（a底图据Stern et al., 1996，b ~ d底图据Altherr et al., 2002；Kaygusuz et al., 2008）
Figure 6.
图 7 吾和玛高分异I型花岗岩 Sr−Ba (a)和 La−(La/Yb)N (b)关系图及分离结晶趋势（矿物分离结晶趋势线据Rudnick et al., 1995）
Figure 7.
表 1 吾和玛花岗质岩石样品(P006-1-TW1)LA-ICP-MS锆石 U−Th−Pb分析结果
Table 1. LA-ICP-MS zircon U−Th−Pb isotopic analyses for the sample (P006-1-TW1) from Wuhema granitoid rock
测点号 同位素比值 年龄/Ma 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 208Pb/232Th 1σ 206Pb/238U 1σ 1 0.06784 0.00395 0.31801 0.01833 0.034 0.00066 0.01486 0.0006 215.6 4.1 2 0.05287 0.0019 0.26214 0.00959 0.03596 0.00058 0.01137 0.00031 227.7 3.58 3 0.05818 0.00189 0.29154 0.00969 0.03635 0.00058 0.01273 0.00029 230.2 3.58 4 0.05088 0.0023 0.22936 0.01042 0.0327 0.00055 0.00826 0.00035 207.4 3.46 5 0.05048 0.00162 0.24334 0.00801 0.03497 0.00055 0.00969 0.00022 221.5 3.4 6 0.05288 0.0022 0.24881 0.01043 0.03413 0.00057 0.01042 0.00028 216.3 3.53 7 0.05986 0.00209 0.27428 0.00971 0.03323 0.00054 0.01053 0.00028 210.8 3.36 8 0.05236 0.00302 0.24352 0.01396 0.03374 0.00062 0.01108 0.00039 213.9 3.86 9 0.04904 0.00201 0.22609 0.00933 0.03344 0.00055 0.00955 0.00027 212.1 3.42 10 0.05268 0.00206 0.24899 0.00984 0.03428 0.00056 0.01051 0.0003 217.3 3.49 11 0.06386 0.0023 0.27834 0.01013 0.03161 0.00052 0.01295 0.00033 200.6 3.23 12 0.09867 0.00235 0.48867 0.01222 0.03592 0.00056 0.01552 0.00029 227.5 3.48 13 0.05043 0.00351 0.2396 0.01657 0.03446 0.00066 0.0113 0.00036 218.4 4.11 14 0.05113 0.0023 0.24263 0.01099 0.03442 0.00058 0.01022 0.0003 218.2 3.59 15 0.06856 0.00256 0.2753 0.01036 0.02913 0.00048 0.01268 0.00035 185.1 3.02 16 0.05385 0.00278 0.25171 0.01297 0.03391 0.0006 0.01174 0.00037 215 3.73 17 0.05591 0.0023 0.24805 0.0103 0.03218 0.00053 0.01085 0.00032 204.2 3.31 18 0.05553 0.00258 0.2759 0.01287 0.03604 0.00062 0.01222 0.00046 228.2 3.84 19 0.05093 0.002 0.2454 0.00975 0.03495 0.00057 0.00982 0.00031 221.5 3.56 20 0.33927 0.00537 3.34562 0.05896 0.07153 0.00111 0.18423 0.00285 445.4 6.67 
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表 2 吾和玛花岗质岩石锆石Lu-Hf同位素分析结果
Table 2. Zircon Lu-Hf isotopic data of Wuhema granitoid rock
测点号 176Yb/177Hf 176Lu/177Hf ±2σ (176Hf/177Hf )i εHf(t) ±1σ TDM2/Ga Hf-BD02 0.023024 0.000850 0.000018 0.282764 -0.27 0.03 1.10 Hf-BD03 0.029089 0.001110 0.000022 0.282736 -1.28 0.03 1.17 Hf-BD04 0.022269 0.000861 0.000022 0.282727 -1.59 0.03 1.19 Hf-BD05 0.037337 0.001418 0.000022 0.282807 1.24 0.03 1.01 Hf-BD06 0.021790 0.000849 0.000019 0.282700 -2.54 0.03 1.25 Hf-BD07 0.036377 0.001300 0.000021 0.282809 1.32 0.03 1.00 Hf-BD08 0.037459 0.001345 0.000022 0.282765 -0.26 0.03 1.10 Hf-BD09 0.024019 0.000934 0.000021 0.282734 -1.35 0.03 1.17 Hf-BD10 0.022585 0.000857 0.000021 0.282711 -2.15 0.03 1.22 Hf-BD11 0.032601 0.001179 0.000024 0.282753 -0.68 0.03 1.13 注: εHf(t)={［(176Hf/177Hf)样品-(176Lu/177Hf)样品×(eλt－1)］/［(176Hf/177Hf)CHUR－(176Lu/177Hf)CHUR×(eλt–1)］－1)}×10000，tDM1=1/λ×ln{1 +［(176Hf/177Hf)样品-(176Lu/177Hf)DM］/［(176Lu/177Hf)样品-(176Lu/177Hf)DM］}，TDM2 = TDM1-( TDM1－t)×［(fcc–f样品)/(fcc–fDM)］，(176Lu/177Hf) CHUR =0. 0332，(176Hf/177Hf) CHUR= 0.282772(据Blichert-Toft et al.,1999) ； (176Lu/177Hf) DM = 0. 0384, (176Hf/177Hf)DM = 0.28325 (据Griffin et al.,2000) , fcc, f样品和fDM分别代表陆壳、样品和亏损地幔的fLu/Hf值，t表示锆石结晶年龄，λ = 1. 867 × 10−11/a
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表 3 吾和玛花岗质岩石全岩主量、微量和稀土元素组成分析结果
Table 3. Whole-rock major, trace and rare earth elements of the Wuhema granitoid rock
样品编号 PM006-2-YH2 PM006-2-YH1 PM006-3-YH1 PM006-3-YH2 PM006-4-YH1 PM006-4-YH2 PM006-5-YH1 PM006-5-YH2 SiO2 75.5 74.0 76.8 76.8 76.4 77.3 76.5 76.7 TiO2 0.075 0.22 0.10 0.10 0.08 0.06 0.11 0.07 Al2O3 12.26 13.4 12.22 12.31 12.23 12.27 12.31 12.43 Fe2O3 1.50 0.34 0.14 0.05 0.23 0.09 0.05 0.03 FeO 1.30 1.40 1.4 1.30 1.4 0.80 1.50 1.10 MnO 0.02 0.04 0.03 0.03 0.03 0.02 0.04 0.04 MgO 0.16 0.25 0.12 0.11 0.11 0.06 0.15 0.07 CaO 0.65 1.2 0.80 0.70 0.70 0.50 0.70 0.70 Na2O 3.49 3.5 3.41 3.14 3.13 3.34 3.4 3.36 K2O 4.43 4.32 4.44 4.81 4.82 4.88 4.59 4.82 P2O5 0.012 0.06 0.02 0.02 0.02 0.01 0.02 0.02 H2O 4.53 4.32 4.78 4.8 4.75 4.91 4.74 4.78 总计 99.99 100 100 100.01 100 99.98 100.01 99.98 A/CNK 1.05 1.07 1.03 1.06 1.06 1.05 1.04 1.04 烧失量 0.60 1.25 0.58 0.65 0.95 0.69 0.59 0.70 Mg# 10 21 12 13 11 11 15 10 DI 92 93 93 93 93 95 90 94 Ta 1.56 0.85 0.9 0.81 0.85 0.93 1.16 0.99 U 10.33 5.38 7.08 8.73 10.9 7.78 8.53 8.81 Ba 27.3 89 100 78.3 63.4 54.6 66.2 37.9 Cs 12.74 14.6 11.6 17.7 19.1 14.5 19.8 14.2 Zr 77.95 135 88.6 104 85.2 88 95.8 76.3 Hf 3.34 4.82 3.76 4.44 4.01 4.12 4.48 3.53 Rb 317.82 258 312 348 397 418 384 429 Th 33.63 28.1 30.7 28 31.9 21.2 35.2 31.3 Sr 26.47 29 39.2 33 28.4 21.9 32.8 21.6 Sn 3.72 7.34 6.12 14.7 9.8 6.04 10.4 9.54 Nb 7.84 12.6 12.4 12.5 12.6 13 14.2 13 La 13.43 23.4 15.4 7.66 16.2 7.12 17 12.5 Ce 74.91 46.2 34.7 19.2 38.6 15.5 37.6 30.2 Pr 4.03 5.26 4.16 2.25 4.51 1.61 4.42 3.66 Nd 16.08 19.8 16.5 8.49 17.4 5.85 17 13.9 Sm 4.27 4.42 4.55 2.18 4.46 1.49 4.87 3.44 Eu 0.16 0.61 0.21 0.17 0.12 0.067 0.17 0.1 Gd 4.39 4.11 4.56 2.61 4.41 2.12 5.12 3.57 Tb 0.73 0.65 0.76 0.48 0.75 0.4 0.9 0.58 Dy 4.93 3.89 4.73 3.3 4.82 2.75 5.75 3.54 Ho 1.05 0.83 0.98 0.73 0.99 0.61 1.24 0.76 Er 3.33 2.43 2.84 2.3 3.01 1.9 3.66 2.27 Tm 0.58 0.38 0.44 0.36 0.48 0.31 0.58 0.36 Yb 4.25 2.61 2.94 2.56 3.26 2.16 4.12 2.58 Lu 0.72 0.37 0.43 0.36 0.48 0.3 0.57 0.36 Y 31.41 22.4 26.4 20.8 28 18.4 34.5 21.5 δEu 0.11 0.43 0.14 0.22 0.08 0.12 0.1 0.09 Sr/Y 0.05 0.04 0.05 0.05 0.05 0.05 0.05 0.05 (La/Yb)N 2.27 6.43 3.76 2.15 3.56 2.36 2.96 3.48 La/Yb 3.16 8.97 5.24 2.99 4.97 3.3 4.13 4.84 ΣREE 132.86 114.96 93.2 52.65 99.49 42.19 103 77.82 注：主量元素含量单位为%，微量和稀土元素含量单位为10−6；Mg#=100×Mg2+/(Mg2++TFe2+)；A/CNK=Al2O3/(CaO+Na2O+K2O)；A/NK=Al2O3/(Na2O+K2O)；δEu=2×EuN/(SmN+GdN)；N表示球粒陨石标准化
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