Insights into petrogenesis of the Jálama pluton (Central Iberian Zone, western Spain)

The Jálama pluton (JP) is a Variscan peraluminous granitoid that intruded into low-grade metasediments from the Central Iberian Zone (CIZ). It comprises a sillimanite-bearing two-mica monzogranite in the inner zone, followed by a tourmaline-bearing two-mica monzogranite, and a marginal tourmaline-muscovite leucogranite in the northern half of the pluton. Microgranitoid enclaves and metasedimentary xenoliths occur locally in monzogranites. The change in rock type from the central monzogranite to the marginal leucogranite corresponds to decreasing TiO₂, MgO, FeO, CaO, Sr, Ba, Zr, and ΣREE, and increasing SiO₂, Na₂O, P₂O₅, Rb, Li, Cs, Ta, Sn, and W. Fe/(Fe+Mg) ratios in biotite, muscovite and tourmaline increase with increasing Fe/(Fe+Mg) in bulk rock, suggesting an important control of the bulk-rock composition on mineral chemistry. The high peraluminosity, the low CaO and high P contents, as well as the similarity of <i>ε</i>(Nd)₃₀₀ values in both the granites and metasediments of the southern CIZ constitute strong evidences for a crustal origin of the granite suite, probably by melting of these metasedimentary rocks. Field and petrographic observations, together with mineralogical and geochemical data, suggest that assimilation and mingling/mixing acted in concert with fractional crystallization during the formation of the JP. These processes may also have been important in the evolution of other granitoids from this region.

哈拉马深成岩体（Jálama Pluton, JP）是一处华力西期过铝质花岗岩类（Variscan peraluminous granitoid）岩体，侵入于伊比利亚中部带（Central Iberian Zone, CIZ）的低级变质沉积岩中。该岩体内部带为含矽线石二云母二长花岗岩，向北依次过渡为含电气石二云母二长花岗岩，以及岩体北半部的边缘相含电气石白云母淡色花岗岩。二长花岗岩体内局部可见微花岗岩质包体（microgranitoid enclaves）与变质沉积岩捕虏体（metasedimentary xenoliths）。岩石类型从中心二长花岗岩向边缘淡色花岗岩的变化，对应全岩中TiO₂、MgO、FeO、CaO、Sr、Ba、Zr及总稀土元素（ΣREE）含量降低，同时SiO₂、Na₂O、P₂O₅、Rb、Li、Cs、Ta、Sn及W含量升高。黑云母、白云母与电气石的Fe/(Fe+Mg)比值随全岩Fe/(Fe+Mg)比值升高而增大，表明全岩组成对矿物化学组成具有重要控制作用。高过铝度、低CaO含量与高P含量，以及伊比利亚中部带南部花岗岩与变质沉积岩的ε(Nd)₃₀₀值相似性，为该花岗岩套的壳源成因提供了有力证据，其成因可能为上述变质沉积岩的熔融作用。野外与岩相学观测结果，结合矿物学与地球化学数据表明，哈拉马深成岩体形成过程中，分离结晶作用（fractional crystallization）与同化混染、混合/混杂作用（assimilation and mingling/mixing）共同发挥作用。该演化过程或许对该区域其他花岗岩类岩体的演化也具有重要意义。