New garnets for old? Cautionary tales from young mountain belts

摘要

Major and trace elements in garnet may be used to characterize P-Tt paths in orogenic belts, but they are also often used without age constraints to obtain P-T estimates alone. During investigations into the prograde phase of the Alpine-Himalayan Tertiary orogeny, garnets with anomalously old Sm-Nd ages were discovered in three different areas. They occurred in identical parageneses to garnets that grew during Tertiary prograde metamorphism. Ages > 500 Ma were determined for homogenized garnets from the Garhwal Himalaya, while garnets with distinctive growth zonation from the Betic Cordillera (S. Spain) gave an age of 235.1 +- 1.7 Ma. Although the Garhwal data coincides with a recognized magmatic and metamorphic event, the 235.1 Ma age from the Betics at first sight appears one of the more unlikely times for garnet growth. Duplicate analyses precisely data growth of the garnet cores, which X-ray maps reveal to be entirely distinct from the rims, implying a hiatus before resorption and rim development. This initial garnet growth may be linked to early crustal extension during Pangaea break-up. These results reaffirm the value of garnet in the integrated study of polymetamorphic orogenic belts. At the same time, the data show that potential for unravelling the history of these complex zones, and strike a cautionary note for thermobarometry.