The Shanderman eclogites: a Late Carboniferous high-pressure event in the NW Talesh Mountains (NW Iran)

The Shanderman Metamorphic Complex, exposed along the Caspian foothills of the Talesh Mountain, western Alborz, Iran, has
always been interpreted as an ophiolitic fragment of the Palaeotethys Ocean. According to our new data, this unit consists
of metamorphic rocks mainly represented by garnet–staurolite micaschists with large bodies of metabasites containing well-preserved
eclogitic-phase assemblages. The Shanderman Complex (SC) was later intruded at middle crustal levels by intermediate–basic
intrusive bodies. New Ar/Ar ages of paragonitic white micas in equilibrium with the high-pressure assemblages have given a
Late Carboniferous age (315±9 Ma). Our new data suggest that the SC was equilibrated in high-pressure conditions during an
orogenic event that predates the Eo-Cimmerian orogeny by more than 100 Ma and that may be tentatively ascribed to the Variscan
orogeny sensu latu. We suggest that the Shanderman Complex represents a fragment of the Upper Palaeozoic European continental crust. The occurrence
of eclogites in these regions can be explained by two different hypotheses: (1) the SC high-pressure rocks can be related
to the accretion of Gondwana-related Transcauscasian–Moesian microplate to the southern margin of Eurasia; or (2) the SC eclogites
can represent a fragment of the Late Palaeozoic ‘Variscan belt’ sensu latu of central Europe, which has been translated eastwards during Permian along a dextral megashear zone taking from a Pangea-B
to a Pangea-A plate configuration. This metamorphic unit was stacked southwards on the northern edge of the Iran Plate during
the Eo-Cimmerian events occurring at the end of the Triassic. The eclogite-bearing basement of the SC was finally exhumed
at the end of the Eo-Cimmerian orogeny, as suggested by the composition of the basal layers of the Shemshak Group dated here
Middle Jurassic, that cover the crystalline rocks of the SC along a regional non-conformity. The SC was probably displaced
further southwards during the Mesozoic opening of the South Caspian Basin and the Tertiary thrust stacking and dextral shearing
accompanying the formation of the Alborz intracontinental belt.

沙德曼变质杂岩，位于伊朗西部阿勒博兹山脉的塔勒斯山脚下的里海丘陵地带，长期以来一直被视为古太特提斯洋的蛇绿岩碎片。根据我们的新数据，该单元主要由石榴石-十字石片麻岩组成，并含有大量包含保存完好的生态岩相组合的变质玄武岩。沙德曼杂岩（SC）后来在中地壳水平上被中基性侵入体侵入。与高压组合处于平衡状态的角闪石白云母的新Ar/Ar年龄测定出晚石炭纪的年龄（315±9 Ma）。我们的新数据表明，SC在早喜马拉雅造山运动之前超过1亿年的造山事件中，在高压条件下达到平衡状态，这一事件可能暂且归因于广义的瓦利斯造山运动。我们认为，沙德曼杂岩代表上古生代欧洲大陆地壳的碎片。这些地区生态岩的出现可以通过两种不同的假说来解释：（1）SC高压岩可以与冈瓦纳相关的外高加索-摩尔达维亚微板块向欧亚大陆南部边缘的拼贴有关；或者（2）SC的生态岩可以代表广义的欧洲中部晚古生代“瓦利斯带”的碎片，该带在二叠纪期间沿右旋大剪切带向东迁移，从泛古大陆B板块转变为泛古大陆A板块的构架。这一变质单元在早喜马拉雅事件中堆积在伊朗板块的北部边缘，这些事件发生在三叠纪末期。SC携带生态岩的基底最终在早喜马拉雅造山运动的末期被抬升，正如在这里测年的中侏罗纪的谢姆沙克组底部层位所表明的，这些层位沿着区域非整合面覆盖了SC的结晶岩。SC可能在侏罗纪南里海盆地的张开以及阿尔博兹内陆造山带的形成过程中伴随的第三纪逆冲堆积和右旋剪切进一步向南移动。