RELATING PLAGIOGRANITE PODS IN THE TROODOS OPHIOLITE TO SILICIC ERUPTIVES IN THE MID-OCEAN RIDGE ENVIRONMENT

Pods and veins of plagiogranite are ubiquitously found between sheeted dikes and gabbros in many ophiolites. How these form from basaltic source magmas remains enigmatic. Somewhat comparable, dacitic and even rhyolitic lavas are also erupted in some MOR settings. However, how or if the silicic plutonic rocks with >70 wt.% SiO2 are related to these felsic volcanics, is another unanswered question. In fact, it is not clear if the plutonic and volcanic rocks form via the same petrogenetic processes.

We collected 14 samples along a 700m transect from normal gabbros out to sheeted dikes in the eastern Troodos Ophiolite (near Agros, Cyprus). Silica contents increase up to 75 wt. % and MgO decreases with distance away from the gabbros. We hypothesize that this differentiation sequence reflects a long-term thermal migration process operating along the horizontal temperature gradient of the axial magma lens. To test this, we performed Fe isotope analysis by MC-ICP-MS. The data show a steady increase in the δ56Fe isotopic ratio with distance from the gabbros, consistent with predictions of differentiation occurring within a temperature gradient. Specifically, samples near the dikes have a heavy isotope signature, whereas samples approaching the gabbros have a light isotope signature. Preliminary Si isotope data also show δ30Si increases away from the gabbros.

Notably, our analyses of δ56Fe in 3 silicic MOR suites at ridge crest discontinutities (Galapagos, JdF-Blanco and 9°N EPR) show the same trend of increasingly heavy ratios with increasing silica content. This suggests that these two silicic rock types may indeed be related. The silicic volcanic rocks may represent wholesale melting plagiogranite bodies and subsequent eruption as a consequence of the intrusion of new batches of basalt into older, cooler crust at overlapping spreading centers and transform intersections. We suggest bodies of plagiogranite may ubiquitously exist at the base of the sheeted dikes in most ocean crust but are rarely seen. Understanding the formation process of these anomalous plagiogranite bodies could also shed light on the formation of primitive continental crust.