LA-ICP-MS zircon U-Pb dating and geochemical characteristics of the Jiaogou syenite in western Henan Province and its geological implications
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摘要:
豫西纸房-黄庄地区是东秦岭造山带印支期碱性侵入岩规模最大的出露区，对其中的焦沟岩体进行了LA-ICP-MS锆石U-Pb定年和元素地球化学分析，探讨其源区特征及深部动力学背景。焦沟岩体定年样品JG1701为霓辉正长岩，出现了约2.6 Ga、约2.5 Ga、约2.4 Ga、约2.3 Ga、约2.1 Ga、约1.9 Ga和约243 Ma 7个年龄组，形成了锆石年龄谱，最晚一组6颗锆石测点的206Pb/238U年龄为243.2±4.8 Ma，表明其形成于中三叠世。焦沟岩体的Na2O+K2O含量为11.50%~14.01%，碱度率（AR）和K2O/Na2O值分别为3.92~9.17和3.78~10.25，属于碱性-过碱性系列，且具有超钾质岩石的特征。焦沟岩体的稀土元素总量介于176×10-6~315×10-6之间，（La/Yb）N范围为9.51~20.43，稀土元素配分模式具有右倾、轻稀土元素富集、重稀土元素亏损和无明显负Eu异常的特征。焦沟岩体强烈富集大离子亲石元素、亏损高场强元素，在微量元素蛛网图中显示了一致的Th、Nb、Ta、P、Ti异常谷，以及Ba、K、Zr、Hf异常峰。它的部分熔融源区位于石榴子石-尖晶石过渡带，埋深介于60~80 km之间，残余矿物包括金云母、富钛矿物（如金红石和钛铁矿）、石榴子石和尖晶石。焦沟岩体是幔源富钾岩浆/流体与壳源岩浆/流体库混合活化之后形成的，深部控制过程为岩石圈拆沉作用。
Abstract:The Zhifang-Huangzhuang(ZH)area in western Henan Province has the largest outcrop of Indosinian alkaline intrusives in eastern Qinling orogenic belt.In this paper, the LA-ICP-MS zircon U-Pb ages as well as major elements and trace elements of the Jiaogou syenite were tested in order to obtain new constraint on formation time, rock source of partial melting and tectonic process of syenites in the ZH area.The U-Pb dating sample JG1701 is aegirine-augite syenite, and zircon age spectrum is composed of 7 age groups(~2.6 Ga, ~2.5 Ga, ~2.4 Ga, ~2.3 Ga, ~2.1 Ga, ~1.9 Ga and~243 Ma).The weighted mean age of 6 zircon spots form the last group is 243.2±4.8 Ma, which suggests that it was formed in Middle Triassic.The Jiaogou syenite is ultrapotassic rocks and belongs to alkaline-peralkaline series with higher Na2O+K2O content(11.50%~14.01%), alkalinity ratios(3.92~9.17)and K2O/Na2O ratios(3.78~10.25).The total rare earth elements(REEs)and(La/Yb)N ratios of the Jiaogou syenite are 176×10-6~315×10-6 and 9.51~20.43, which shows the characteristics of LREE enrichment and HREE depletion without obvious Eu negative anomaly.They are enriched in large ion lithophile elements and depleted in high field strength elements, exhibiting negative anomalies of Th, Nb, Ta, P and Ti and positive anomalies of Ba, K, Zr and Hf in spider diagram.The source rock of the Jiaogou syenite was situated in the garnet-spinel transition zone at the depth of about 60~80 km, and its residual phases included phlogopite, Ti-enriched minerals such as rutile and ilmenite, garnet and spinel.The Jiaogou syenite is the mixed product of mantle-derived K-enriched magmas/fluids and crust-derived magmas/fluids, and it was controlled by regional lithosphere delamination in Middle Triassic.
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图 2 焦沟岩体岩石照片(矿物缩写代号据参考文献[11])
a—正长岩脉侵入熊耳群安山玢岩；b—致密块状的焦沟霓辉正长岩；c—霓辉正长岩中的霓辉石、霓石和正长石(单偏光)。Agt—霓辉石；Aeg—霓石；Or—正长石
Figure 2. Petrographs of the Jiaogou syenite
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图 3 焦沟岩体定年样品JG1701锆石阴极发光图像
(锆石年龄大于1000 Ma时标注207Pb/206Pb年龄，锆石年龄小于1000 Ma时标注206Pb/238U年龄)
Figure 3. Cathodoluminescence images of zircons in sample JG1701 from the Jiaogou syenite
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图 4 焦沟岩体LA-ICP-MS锆石U-Pb年龄谐和图
a—样品JG1701中24颗锆石的U-Pb谐和图；b—样品JG1701中15颗锆石(不含测点JG1701-03、06、07、09、11、13、16、22和23)的U-Pb谐和图；c—样品JG1701中6颗锆石(JG1701-04、10、14、15、21和24)的U-Pb谐和图；d—样品JG1701中6颗锆石测点的206Pb/238U年龄加权平均值
Figure 4. Concordia diagrams of LA-ICP-MS zircon U-Pb age for the Jiaogou syenite
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图 6 焦沟岩体SiO2-(Na2O+K2O)(a)、AR-SiO2(b)和Na2O -K2O(c)图解
a—SiO2-(Na2O+K2O)图解(底图据参考文献[14]，碱性与亚碱性系列界线据参考文献[15])；1—橄榄辉长岩；2—辉长岩；3—辉长闪长岩；4—闪长岩；5—花岗闪长岩；6—花岗岩；7—副长石辉长岩；8—二长辉长岩；9—二长闪长岩；10—二长岩；11—石英二长岩；12—副长石岩；13—副长石二长闪长岩；14—副长石二长正长岩；15—正长岩；16—副长石正长岩；b—AR-SiO2图解(底图据参考文献[16]；c—Na2O-K2O图解(底图据参考文献[17]
Figure 6. Diagrams of SiO2-(Na2O+K2O)(a), AR-SiO2 (b)and Na2O-K2O(c)for the Jiaogou syenite
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表 1 焦沟岩体样品(JG1701)LA-ICP-MS锆石U-Th-Pb定年结果
Table 1 LA-ICP-MS U-Th-Pb dating results of zircons in sample JG1701 from the Jiaogou syenite
测点编号 Pb/10-6 Th/10-6 U/10-6 Th/U 207Pb/206Pb比值 1σ 207Pb/235U比值 1σ 206Pb/238U比值 1σ 207Pb/206Pb年龄/Ma 1σ/Ma 207Pb/235U年龄/Ma 1σ/Ma 206Pb/238U年龄/Ma 1σ/Ma 谐和度 JG1701-01 305 28.8 931 0.03 0.1175 0.0020 5.3110 0.0920 0.3267 0.0026 1918 30 1871 15 1823 13 97% JG1701-02 250 148 446 0.33 0.1282 0.0027 5.7031 0.1533 0.3199 0.0038 2074 37 1932 23 1789 18 92% JG1701-03 39.0 360 525 0.69 0.0766 0.0067 0.2720 0.0256 0.0249 0.0004 1122 176 244.3 20.4 158.6 2.8 57% JG1701-04 81 414 1105 0.37 0.0593 0.0024 0.3191 0.0126 0.0392 0.0005 576.0 88.9 281.2 9.7 248.0 3.2 87% JG1701-05 625 107 1222 0.09 0.1624 0.0024 8.5674 0.1347 0.3816 0.0028 2481 24 2293 14 2084 13 90% JG1701-06 281 929 1811 0.51 0.1174 0.0098 0.6708 0.0632 0.0386 0.0005 1916 150 521.2 38.4 244.1 3.1 27% JG1701-07 105 649 761 0.85 0.1304 0.0049 0.4880 0.0187 0.0272 0.0003 2103 66 403.6 12.8 172.8 2.1 19% JG1701-08 466 337 471 0.72 0.1576 0.0026 8.9714 0.1760 0.4110 0.0039 2431 28 2335 18 2219 18 94% JG1701-09 189 420 939 0.45 0.1661 0.0059 1.0030 0.0428 0.0431 0.0006 2520 66 705.3 21.7 272.0 3.7 11% JG1701-10 144 990 1835 0.54 0.0551 0.0016 0.2903 0.0078 0.0383 0.0004 416.7 63.0 258.8 6.2 242.5 2.3 93% JG1701-11 690 2048 1548 1.32 0.2408 0.0111 1.7398 0.1079 0.0491 0.0008 3125 73 1023 40 309.3 5.0 -8% JG1701-12 542 351 704 0.50 0.1488 0.0031 7.8451 0.1879 0.3780 0.0032 2332 36 2213 22 2067 15 93% JG1701-13 51.4 540 720 0.75 0.0688 0.0038 0.2207 0.0112 0.0236 0.0003 900.0 118.7 202.5 9.3 150.4 1.8 70% JG1701-14 67.8 352 1168 0.30 0.0574 0.0022 0.3112 0.0117 0.0394 0.0004 505.6 83.3 275.1 9.0 249.1 2.7 90% JG1701-15 116 670 1844 0.36 0.0542 0.0017 0.2935 0.0102 0.0390 0.0004 388.9 72.2 261.3 8.0 246.4 2.4 94% JG1701-16 142 509 1331 0.38 0.0904 0.0040 0.4854 0.0218 0.0389 0.0003 1435 85 401.8 14.9 245.9 1.9 51% JG1701-17 1081 900 779 1.15 0.1501 0.0023 8.8056 0.1785 0.4216 0.0051 2347 26 2318 19 2268 23 97% JG1701-18 343 181 527 0.34 0.1277 0.0022 6.6459 0.1124 0.3763 0.0029 2066 30 2065 15 2059 14 99% JG1701-19 607 185 849 0.22 0.1682 0.0028 11.5544 0.2248 0.4950 0.0049 2540 23 2569 18 2592 21 99% JG1701-20 823 537 730 0.74 0.1741 0.0028 10.9774 0.2283 0.4547 0.0061 2597 27 2521 19 2416 27 95% JG1701-21 203 1941 1464 1.33 0.0493 0.0013 0.2568 0.0070 0.0377 0.0004 161.2 67.6 232.1 5.7 238.3 2.2 97% JG1701-22 56.3 419 566 0.74 0.0910 0.0039 0.3184 0.0129 0.0256 0.0003 1448 81 280.7 10.0 163.1 2.1 47% JG1701-23 125 1183 1384 0.85 0.1022 0.0022 0.9527 0.0392 0.0652 0.0021 1665 36 679.5 20.4 407.4 12.8 49% JG1701-24 145 1304 1388 0.94 0.0499 0.0016 0.2597 0.0081 0.0378 0.0004 190.8 69.4 234.5 6.5 239.1 2.3 98% 
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表 2 焦沟岩体主量、微量和稀土元素分析结果
Table 2 Results of major, trace and rare earth elements from the Jiaogou syenite
样品号 JG0111A S30116A S30216D S30316E JG1701 JG1702 JG1703 SiO2 56.95 60.51 59.80 60.95 59.89 58.87 62.15 TiO2 0.75 0.46 0.47 0.50 0.47 0.62 0.60 Al2O3 14.10 17.35 17.07 16.06 15.11 14.49 15.92 TFe2O3 8.07 5.25 5.26 4.58 5.90 8.13 5.60 MnO 0.19 0.01 0.02 0.09 0.10 0.13 0.03 MgO 0.62 0.24 0.19 0.36 0.65 0.74 0.26 CaO 5.27 0.31 0.61 1.38 1.93 2.25 0.59 Na2O 1.90 1.18 1.30 1.56 2.78 2.40 1.24 K2O 9.60 12.10 12.20 12.45 10.50 10.45 11.70 P2O5 0.26 0.08 0.09 0.12 0.16 0.24 0.14 烧失量 0.94 1.51 1.85 1.03 1.03 1.02 1.24 总量 98.65 99.00 98.86 99.08 98.52 99.34 99.47 AR 3.92 7.06 7.46 9.17 8.06 7.61 8.25 Ba 11103 11193 13790 7790 8120 8380 3260 Rb 214 303 297 297 256 257 325 Th 10.6 8.61 20.4 7.21 4.71 9.19 10.45 Nb 7.70 15.5 19.2 14.8 12.4 14.5 23.9 Ta 0.3 0.6 0.6 0.6 0.6 0.6 1.2 Sr 1530 1150 1150 866 836 1165 328 Zr 378 322 366 253 256 323 285 Hf 9.6 7.6 8.4 6.4 7.3 8.4 7.0 Y 35.5 20.4 28.3 19.5 17.7 40.8 31.3 La 43.6 32.6 44.7 29.3 69.4 54.1 65.4 Ce 101 67.2 80.1 75.6 93.7 107 131 Pr 13.1 10.3 12.3 10.3 12.7 13.0 17.7 Nd 58.5 41.0 47.5 40.4 44.1 51.2 64.0 Sm 12.2 7.66 8.53 7.69 7.06 10.3 10.8 Eu 2.89 1.48 1.69 1.26 1.69 2.95 2.11 Gd 9.07 5.25 6.11 5.19 5.24 9.06 7.87 Tb 1.20 0.67 0.85 0.74 0.72 1.31 1.12 Dy 6.66 3.85 4.88 4.07 3.72 7.36 6.30 Ho 1.19 0.77 0.93 0.81 0.74 1.45 1.20 Er 3.27 2.18 2.54 2.05 1.93 3.98 3.20 Tm 0.48 0.31 0.35 0.32 0.30 0.59 0.45 Yb 3.09 2.07 2.37 2.02 2.29 3.82 3.05 Lu 0.46 0.32 0.35 0.32 0.40 0.60 0.52 ΣREE 256 176 213 180 244 266 315 (La/Yb)N 9.51 10.6 12.7 9.78 20.4 9.55 14.5 δEu 0.81 0.68 0.68 0.58 0.81 0.91 0.67 注：元量元素含量单位为%，微量和稀土元素含量为10-6
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