We have been perusing the Corpus of Early Arabic Sources for West Africa History and the reports and studies by archaeologists and wonder if the Garamantian state, in some form, persisted well into the Islamic era. We know from John of Biclar's chronicle that the Garamantes sent an envoy to the Byzantines in 568, which is after the Vandal Wars and after Justinian, according to Procopius, had built churches and promoted Christianity at Ghadames and among the Berbers in Tropolitania. Even though there is no evidence of conversion to Christianity in the 6th century Fazzan, we know the Byzantines at least reached some of their neighbors. This seems to imply some
Then we have a period where we know little about the Fazzan until the 660s, when Uqba b. Nafi arrived and imposed tribute on the Fazzan and Kawar. Just using Arabic sources from the medieval period, one finds references to Qaramantiyyun, Qazan, and Fazzani people as "Sudan" ("blacks") who were described as non-Muslims. Reading al-Tabari on the Zanj Revolt in 9th century Iraq, one finds references to people from the Fazzan among the Zanj (black) slaves. Sahnun, in the 800s, also quoted the opinion of Malik ibn Anas on the Fazzanis, implying that they were still non-Muslim blacks in the late 700 (see the fascinating article by Brunschevig on this). Then, al-Ya'qubi referred to the Fazzan region as ruled by a powerful chief who was always at war with the Mazata Berbers in Wadan.
The Mazata were Ibadites, and probably among the slave-traders described by the same author as operating in Zawila and Kawar. My guess is that Waddan and Germa were the centers of two different polities that dominated the region, and in Waddan and Zawila, the Ibadites and their trading network were powerful enough to be the victim of a raid from the Abbasids in c.762-763. Yet, over a century later, another part of the Fazzan region is described as independent. The polity controlling the section of the Fazzan in opposition to the Mazata at Waddan were likely the Garamantes, whose wars with the Ibadi Mazata would have led to some of them becoming captives sold into slavery abroad.
I think further evidence of this Garamantian state surviving longer can be seen in the remnant populations living at Germa and Tassawa described by al-Idrisi. Al-Idrisi seems to have been confused about the political layout of the land in the pre-Islamic era, but he described the people at these two towns using an irrigation system (perhaps the foggara in a far less intensive manner). My bet is that this remnant of the Garamantes engaged in trans-Saharan salt trade, since al-Muqaddasi described the Qaramatiyyun transact with salt. Maybe the Garamantes focused on the salt trade in the southern Fazzan and perhaps Kawar while the Mazata and other Berbers engaged in the slave trade with their Ibadi partners and Kanem?
Anyway, al-Muqqadasi was the last source I know of to refer to a population called Garamantes (or something approximate). The Garamantes, at least in some form, survived as a distinct people until the end of the 10th or early 11th century, and were likely a heterogeneous Berber population. Islamization of Garama and its surroundings appears to have been complete by this time, too, with archaeologists like Mattingly reporting findings of a central mosque in the ancient Garamante capital.