云南马厂箐富碱斑岩埃达克岩性质的厘定及其成矿意义

马厂箐复式岩体SiO2含量变化于61.56%～71.63%,平均67.30%(≥56%);Al2O3含量变化于13.38%～17.18%,平均15.44%(≥15%);K2O含量变化于3.36%～8.92%,平均5.35%;K2O+Na2O变化于7.75%～11.55%,平均9.08%;K2O/Na2O变化于0.65～4.00,平均1.49,明显具有高钾钙碱性或钾玄岩系列特征;MgO含量变化于0.40%～4.59%,平均1.11%,在R1-R2图解中处于造山晚期和同碰撞期岩浆岩的范围内.地球化学特征显示高场强元素HFSE(Nb、Ta、Ti)相对亏损,Sr含量高(337×10-6～718×10-6),Y主要集中在6.2×10-6～15.8×10-6之间(≤18×10-6),Yb含量变化于0.20×10-6～1.63×10-6之间(≤1.9×10-6),轻重稀土元素强烈分异,且具有明显的轻稀土元素富集的特点,LREE/HREE变化于8.02～24.01,Sr/Y变化于40.5～57.4之间,平均48.2(＞40),La/Yb变化于17.5～105.8之间,平均43.4(＞20),Sc含量变化于2.5×10-6～7.9×10-6之间(＜10×10-6);δEu 变化于0.81～1.38之间,显示出埃达克岩的地球化学特征.马厂箐岩体属于C型埃达克岩中的钾质埃达克岩.马厂箐岩体埃达克岩性质的厘定及其与同一成矿带上的玉龙斑岩体具有相似地球化学特征和形成环境的认识,对于该区地质找矿具有一定的意义.

Identification of adakitic characteristics of Machangjing alkali-rock porphyry in Yunnan Province and its significance to mineralization research

The composition of Machangjing alkali-rich porphyry was analyzed and the data obtained are reported in this paper. The results show that the porphyry has characteristics of potassium-high calc-alkali series or shoshonite series, with SiO2 being 61.56%～71.63%, averaging 67.30% (≥56%); Al2O3 13.38%～17.18%, averaging 15.44% (≥15%); K2O 3.36%～8.92%, averaging 5.35%; K2O+Na2O 7.75%～11.55%, averaging 9.08%; K2O/Na2O 0.65～4.00, averaging 1.49; and MgO 0.40%～4.59%, averaging 1.11%. The R1-R2 diagram suggests that the porphyritic magma was formed during the late orogeny and the syn-collision. Geochemically, the porphyry is relatively depleted in such HFSE as Nb, Ta, Ti and rich in Sr (337×10-6～718×10-6), with Y being 6.2×10-6～15.8×10-6(≤18×10-6) and Yb being 0.20×10-6～1.63×10-6(≤1.9×10-6). The REE have experienced strong differentiation and are thus markedly enriched in LREE. LREE/HREE ratios vary from 8.02 to 24.01, Sr/Y from 40.5 to 57.4, averaging 48.2(>40), La/Yb from 17.5 to 105.8, averaging 43.4(>20), Sc from 2.5×10-6 to 7.9×10-6(<10×10-6), and δEu from 0.81 to 1.38. All these data indicate that the alkali-rich porphyry is geochemically characterized by adakite and belongs to C-type potassic adakite. The Machangjing porphyry is a kind of adakite and geochemically and envoirnmentally similar to the Yulong porphyry within the same ore-forming belt, which is somewhat of significance to geological prospecting.