Die Entstehung von felsischen Magmen in der ozeanischen Kruste: Assimilation-Fraktionierte-Kristallisation versus Aufschmelzen der Kruste

• Main Author: Freund, Sarah
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2014
• Subjects: Magma; Ozeanische Erdkruste; Gesteinsglas; Fraktionierte Kristallisation; Partielle Schmelze; Ophiolith; ddc:550; Antarctic; Pacific; Antarc*
• Online Access: https://opus4.kobv.de/opus4-fau/frontdoor/index/index/docId/4274; https://nbn-resolving.org/urn:nbn:de:bvb:29-opus4-42745; https://opus4.kobv.de/opus4-fau/files/4274/SarahFreund_Dissertation.pdf

The magmatic oceanic crust mainly consists of mafic rock types like plutonic gabbros and volcanic basalts. Comparable small amounts of felsic rocks with >60 wt% SiO2 are known in the oceanic crust in forms of intrusive veins and plagiogranite bodies but only rarely in form of erupted lavas. Plagiogranitic rocks are well known from ophiolite complexes, which represent a typical sequence of rocks formed in an oceanic setting obducted and exposed on land. Ophiolites therefore provide an easy and less expensive opportunity to study oceanic crustal rocks and those magmatic evolution processes. This thesis focuses on the particular information content provided by felsic rocks concerning various processes during crustal formation. The importance and combination of parameters like e.g. crustal thickening, hydrothermal alteration and assimilation of crustal rocks during melt fractionation beneath a segment of the Pacific Antarctic Rise is discussed in chapter 3. I used major-, trace elements and isotope ratios (δ18O) of fresh and young glassy lavas (basalts, andesites, dacites) and mineral compositions to identify and model the amount of crystal fractionation and assimilation and to detect the composition of the assimilated material. The result of this study indicates that distinct amounts (30%) of hydrothermal modified crustal rocks must be assimilated to produce the felsic lavas. I compared several felsic plutonic rock samples (plagiogranites) from two large and well-preserved ophiolite complexes (Troodos and Oman) with each other and with the associated mafic rocks (gabbros, sheeted dykes and lavas) in chapters 4 and 5. Major-, trace elements and isotope ratios (Sr, Nd, Hf) of whole-rocks and mineral analyses (major elements) were used to demonstrate the genetic relation of the felsic intrusives with the majority of the mafic crustal rocks. The result of these studies indicates that the plagiogranites in the Troodos and Oman ophiolite complexes comprise geochemical different groups. The plagiogranite groups are in ...