俯冲带中H_2O和K的深循环:对碰撞后富钾岩浆成因研究的启示

在俯冲过程中形成的超高压单斜辉石 (>5GPa)含一定量的H2 O和K2 O ,它因此能把地球浅部的水和钾携带到地幔深处。超高压单斜辉石在折返过程中以两种方式释放其中的H2 O和K2 O :(1)在低温环境下进入与单斜辉石共生的出溶矿物相如金云母、角闪石、多硅白云母、钾长石等 ;(2 )当温度足够高到发生部分熔融时 ,H2 O和K2 O将进入熔体相 ,形成富钾岩浆。超高压变质岩石能不能在其折返过程中产生部分熔融主要取决于折返的 p T轨迹。俯冲板片部分熔融形成熔体的性质在很大程度上取决于变质岩的部分熔融程度。虽然经受 p <5GPa变质作用的榴辉岩通过极低程度的部分熔融也有可能形成富钾熔体 ,但由于这类榴辉岩所能携带的K2 O和H2 O相对比较少 ,因此不可能产生大规模富钾熔体 ,但可能形成相对富Na的岩浆。超高压单斜辉石 (>5GPa)部分熔融产生的富钾岩浆可以解释碰撞后富钾岩浆的成因 ,这种富钾岩浆的形成深度一般较大 (位于～ 10 0km的地幔 )。而绿辉石部分熔融形成相对富钠熔体的深度相对较浅。从单斜辉石中释放出来的H2 O和K2 O能够合理地解释大陆碰撞后富钾岩浆的成因 ,通过俯冲板片折返过程的脱水和取钾反应 ,俯冲和折返过程与碰撞后岩浆活动密切联系在一起而成为一个整体。

DEEP RECYCLING OF H2O AND K IN SUBDUCTION ZONE:IMPLICATIONS FOR THE GENESIS OF POST-COLLISIONAL K-RICH MAGMATISM

The ultrahigh-pressure (UHP) clinopyroxene formed during subduction contains some amounts of H_2O and K_2O. Such UHP clinopyroxene therefore can bring abundant H_2O and K into deep mantle. However, during exhumation, these UHP clinopyroxene will release H_2O as well as K_2O because clinopyroxene could not contain these elements at low pressures. There are two mechanisms for the release of these two elements: (1) At low temperature, these elements will participate into the phases such as phlogopite, amphibolite, phengite, K-feldspar exsolved from clinopyroxene; (2) When partial melting happened at high temperatures, H_2O and K will participate into melt and form K-rich magma. Whether the UHP clinopyroxene will melt or not mainly depends on its p-T path at exhumation stage, and the properties of the melt produced by this way depend on the degree of partial melting. The partial melting of eclogites formed at pressure <5 GPa also could produce K-rich melt at very low degree of partial melting. However, such eclogites could not possibly contain high H_2O and K_2O, thus the partial melting of such eclogites cannot produce large amounts of K-rich melt, but it is possible to produce abundant Na-rich magma. The K-rich melt that resulted from partial melting of UHP clinopyroxene could account for the genesis of post-collisional K-rich magmatism. Such K-rich magma generally should be formed at depth ～100 km. The partial melting of omphacite, however, occurs at relatively shallow levels in the upper mantle.